1. They stir doctrine to the surface and compel the church to obey Jude 1:3.
Each week our church recites the Apostles' Creed, and to counter the sense that this is just pointless doctrine or dry liturgy, I always introduce our recitation with a contemporary example or culturally relevant application, giving a reason for our affirming the credal "rule of thumb." Last week, I held up a couple of the heterodox teachings of a local cult group, showing how the creed helps us measure these teachings and find them lacking. Dogma is practical, necessary, and false teachers get us sorting that out.
2. They provoke a show of hands.
Thanks to the inevitable picking of sides, we get to see who aligns with heterodox views and who doesn't.
3. They help us sort out who is willing to trade truth for the zeitgeist.
When the inevitable mudslinging occurs, we get to see who's more concerned with the truth, popular acceptance be danged, and who thinks the church's chief concern on doctrinal matters ought to be better appealing to the lost or to disaffected, doubting evangelicals. We're to be merciful to the naive and doubting, not malleable to them.
4. They help us discover who's really talking and who's really sniping.
I think, in fact, we are now discovering in the current Bell-brouhaha who the real flippant dismissers and sideline carpers are, and it's not the guys writing long, detailed reviews. If you want to see a whole mess of pots yelling "black!" at kettles, check out the comments on some of the emergent or Christian "satire" blogs. A whole lot of insults for Piper, DeYoung, Taylor, et.al., a lot of handwringing about perceived "tone," and very little, if any, serious engagement with the issues involved. While those mean ol' YRR bullies are posting on theology, interacting with the teachings and texts, citing historical examples, and flat-out doin' work, the defenders are just spending a lot of time doing what they claim to decry: insulting and complaining.
5. By identifying themselves, even if unwittingly, as outside orthodoxy, they help make the church stronger.
All of these implications together may affect a pruning of the church, but the strength of the church is not in numbers, but the truth.