Sooo... cynical and negative answer ahead. Look away now if you don't want to know the results. Also, all of this is just my two cents, and I just happen to disagree on this instance.
I think the reason for disallowing in-game teams is that the RMT is clogged as it is, with so many teams going un-rated/ unanswered. I think people would be even less inclined to answer an in-game team when it really doesn't matter. You can beat the game with any combination of Pokemon as long as you a) over train or b) use a ton of items.
Case in point, I've seen someone on Youtube take on Platinum (or D/P or HGSS; it was a Gen IV game) with just a Magikarp and a truck load of X items.
If people are having a problem with a specific gym leader, they can always ask on DB or, as is more often the case, look at the advice on a question that's asked the same thing.
If one was savvy enough, one could just search up what a gym leader has online, and catch something that helps you through that gym; if you have access to this site, you have access to Google.
There's also the "what is a good in-game team for (insert Pokemon game here)" threads, which have some really good answers and provide good advice as to what coverage someone should have when going through a specific game.
I also think if people could just post their team on this site and get it rated/ ranked/ modified by others, it's defeating the point of beating the game yourself. There are already so many help sites out there, like Marriland, Bulbapedia and Serebii, that list the movesets, items and abilities of Pokemon that gym leaders/ the main trainers use, people can just type something into Google and get ready-made advice and walkthroughs for their game, and those walkthroughs also advise what Pokemon to use against which Gym leader. You don't need tailor-made advice because gym leaders are the same for every game, unlike wifi battles or even the PWT.
Competitively, there are a lot more things to consider, usually the fact that your opponent isn't computer generated and has an average of 100 IQ, meaning that you can't just cruise on type advantage. You also have to take into account synergy and EVs, which are much more difficult concepts to grasp than "using a Flying type in Erika's gym is a good idea because Flying > Grass".
Also, why Headbutt trees for Staravia when you can get a Zubat in any cave and it'll trash Erika's gym? :)