Blogtor Who is reporting that Moffat says Matt Smith will be back after filming the Gosling film. Source
I've come to think the break wasn't planned, but since it's there Matt Smith took advantage of it by lining up another short-term job. Hey, an actor's gotta eat.
Here's my theory.
Last fall, as Moffat was struggling to finish series 7b he got behind on his Sherlock
planning, and he realized he couldn't go straight from Doctor Who
filming into Sherlock
filming as he'd originally planned. Working backwards from when Peter Jackson had set filming dates on The Hobbit
(late May), Moffat pushed Sherlock
filming off as late as he could -- the first week of March -- to give him time to finish his script, give Gatiss time to finish his script, and for them both to do the pre-production work. Also, moving Sherlock
two months freed up Cumberbatch to take another offer for the Julian Assange film.
We reach January. As Moffat and Gatiss are working on their Sherlock
scripts, Moffat recognizes that he's not going to be ready for post-Anniversary special Doctor Who
filming, especially since the FX house is still going to be working on series 7b through May and
post-production on the Anniversary special. He and Caroline Skinner decide to push off filming for the post-Anniversary episodes (which we know have already been discussed with Smith) to July. (Why July? Series 5 filming began in July. July is a good start date for filming for an April launch.) Now that there's a two month break (May and June), Moffat tells Matt that he can take another project (and a short one!) if he can find one.
This is just a theory, and it makes several assumptions. Sherlock Holmes would tell me not to theorize in this way, as I've invariably biased the evidence.