There are a lot of good arguments for "let them play" but you do run into a real political problem.
"Hey, you lot helped out Bosniaks, why didn't you help Syria? You lot helped stop the civil war in Libya, why not Syria? You lot, using the UN, spent a lot of time and effort helping in Somalia, for good and ill, why not in Syria?" "
This conversation can take some unfriendly and nasty turns. So modern politics, in the globalized political environment, and membership in things like the UN, lead to political moves that might not otherwise be taken due to quid pro quo.
If you do this, or help there, we'll do that, or not do the other.
It goes on constantly.