Drop-in has a large chance to disrupt a game though. If there is a trueskill-esque team-maker in H4, and people quit and are replaced in a match, those replacements could totally throw off the balance.
Maybe if it searched for a player of similar skill level, it could work.
Also, a possibility to lower the number of quitters would be to have a certain amount of games you can quit in a day. If you're having a really bad day and reach your quit quota, or are trying to derank (If possible. If so, it would slow you down a lot) you can't quit any more matches.
Just throwing out ideas here.