It might be easier now that Riot requires you to invite a friend if you want to build a team. Previously, many players who wanted mid or top lane would just create their own game because it was quicker for them than trying to join another game. This would result is many, many teams being formed, which stretched the limited support and jungler pool over far more games than it should have.
Some people have said that people leaving lobby if they don't like the team is an issue, but there isn't really much you can do about that. The point of team builder is being able to choose your team composition and role without having to fight others for it, so people should be able to opt out of teams they don't like. Since junglers and supports have such short queue times, they are quite free to be picky with their team and you can't really change that without undermining the mode.
I don't really see Riot adding an IP bonus or anything to support or jungle players in teambuilder. Anyways, support and jungle players are already being rewarded in teambuilder with very short queue times (less than 30s in many cases, while some top and mid lane players wait about 10 minutes to find a team) and the ability to be more picky about which teams they join.
