The free (Express) versions of MSVC, while free, don't have all the feature sets of the non-free and not trivially cheap versions have to offer.
To whit, there is not (that I'm aware of) any "free" version of MSVC that can compile FRED. And making it possible for a free version to be able to do so is a grey can of worms at best. Now, this won't be an issue so much once wx_FRED is functional, but somebody at some point is still going to need to be able to compile and validate features and what not under FRED2Open itself for some time to come.
That's just -one- limitation. If I were to count the amount of changes that would take place in migrating to MSVC 2013 Express from 2008 Team Suite, I imagine there would be a lot of differences.
For the record, I love forward progress. But not when it is a case of "simply for the sake of". While I agree that we need to worry less about supporting ever aging constructs, we can't keep throwing the baby out with the bathwater on every new release of an IDE or platform or language shift. Especially when that can be made already all the more difficult when you factor in multi-platform and varied capabilities of who has access to what.