I stated early in Capaldi's run that I like how Moffat, and before him RTD, gave room for lots of adventures that we don't see on screen. Yet one of the complaints being thrown at Moffat by long standing Whovians is how he treats companions. I'm not talking about the raising of them to focus points of the show. Russell T Davies did that, Moffat just kept it going, and really it's what the show has to do to keep the interest of a modern TV audience.
No, the complaint is those companions that don't stay on the TARDIS, but merely treat it as something they can jump on occasionally when they fancy an adventure. Clara personifies this, having barely spent any time just living on board, but latter day Amy and Rory were just as guilty. But this is a symptom of a Doctor that has great control of his ship. Something that wasn't always the case.
Barbara and Ian were only along for the ride because they had to be. If the Doctor could have taken them home at the end of 'An Unearthly Child' they'd have gotten off the TARDIS right there and then, thankful for their lives after escaping Neanderthal man. But the Doctor had no real control over the TARDIS so they were stuck, and only made it home by nicking a different time machine. Even after the Doctor got decent control of the TARDIS this was the status quo for the entire run of Classic Who. It became a case of his companions chose to travel with him for an extended period of time. Even RTD kept this up, though he did introduce the idea of the occasional visit home.
However, between Classic's cancellation in 1989 and the return in 2005 there was something Who fans now refer to as 'The Wilderness Years'. Sure there was The McGann film but other than that, Who only existed as Novels, and in 1998 Big Finish joined in with their audio plays. The approach was generally simple, you find a gap between serials and you shove in some extra adventures. In fact it makes some of the relationships make a lot more sense. Remember how Peri was extremely attached to the Fifth Doctor, and only seemed to initially hang around the Sixth because of that previous connection? But they only had two adventures together, and on the second he died. That's a bit much? Well now there's nearly fifteen adventures between 'Planet of Fire' and 'The Caves of Androzani'. Hard done by companions like Liz Shaw get a decent share of time too.
But you can only insert adventures between existing serials where gaps are obvious, but that ties you down to the existing companions. Based on the show alone, you'd struggle to tell a story of the Second Doctor without Jamie tagging along. Not to mention that each of the companions has their intro and outro, a backstory and probably a serial directly dealing with that. Sure you can expand on that, and sometimes that's been done to great effect, but that character's stories are told, Big Finish and Virgin Books were only able to embellish them. Sometimes you need something more.
The Sixth and Seventh Doctors have it the easiest. Seven has the unenviable position of being the Doctor when the cancellation axe fell, so he had a long future ahead of him. Well up to 1996, but even then we just see his regeneration, and he looked older and that means a lot of time passed for him.
Meanwhile Six has a very undefinable time between the end of 'Trial of a Timelord' and the beginning of 'Time and the Rani' that can give writers a huge scope of time to play with for extra adventures. Add in there some timey-wimey that the Doctor meets Mel long after she's experienced their first encounter and writers have a lot of time to insert extra adventures. Hell, Big Finish have added at least three companions between him dropping future Mel off and him getting round to her first meeting.
The Fourth has the handy moment when he leaves Sarah-Jane, goes off to Gallifrey for 'The Deadly Assassin' and then lets Leela jump aboard. Whose to say those three adventures were straight after each other?
But the others get a little more complicated. You can go earlier than 'An Unearthly Child' for the First Doctor, but as I've discussed previously, it's Barbara and Ian jumping on-board that really make the Doctor, The Doctor.
The Second Doctor has Season 6B, which is about perfect, but The Doctor is very much in the employ of the CIA there. Five is an outright nightmare to not include one of the six companions he had with him, as one was always present as the new one was introduced. Big Finish had to resort to dropping a second companion into that period of extended travels with Peri I mentioned earlier.
The trick was finding the wiggle room, and sometimes it was a very tight fit. Moffat is explicitly giving writers of the expanded universe not just a bit of space to wiggle in, but their very own rooms in what is increasingly becoming a mansion of space. God forbid that Capaldi only does one season (in fact we're already past the point of that even being a possibility), but theoretically if he did, the fact that nearly every episode has him picking up and dropping Clara off gives writers who want to write about Season 8 Twelfth Doctor, but not Clara, wads of room to play in.
It's exactly the same deal when Amy and Rory take their break from the TARDIS. 'Dinosaurs on a Spaceship' is explicit with this very conceit with the Doctor inviting Queen Nefertiti and John Riddell to join in, both having experienced their own adventures with the Doctor and getting the offer to ride in the TARDIS for a bit. Making it quite clear that while it might only be a few weeks between the Doctor's visits for the Ponds, it could be far longer for the Time Lord.
RTD did it too. For Tennant's last year, he shed any companions and went solo. We saw three of those adventures on TV, and fourth was shown digitally (Dreamland). Last year that huge chunk of time was used by Moffat himself for the Tenth Doctor's appearance in “Day of the Doctor” and Titan Comics are using it for their own series, though they also seem to have given the Doctor a proper companion, which is sort of defeating the point of that period in time.
It's understandable why. Both Davies and Moffat first got involved in Who during those Wilderness years. They're used to finding the cracks, and now they're seeding them for future writers. Sometimes you want something that is unique to your own, and for that you need a few gaps scattered around.
The novels and Big Finish have given us companions like Bernice Summerfield, who for those in the know rates as one highest companions. It leaves the field wide open for more people to come up with characters like Evelyn Smith and Frobisher, characters that are both great and would never get okayed by the BBC in one of their premier TV shows. Moffat's doing that, maybe a little too much, but when we're a few years removed and we want new stories of Matt Smith or Peter Capaldi's Doctors we'll be very glad of it.