It's very different to ask for optimal performance and to simply ask for doing something useful. As Bourne_Endeavor pointed out, the only reason threads like that exist is that there are people actively defending sub-par performance in the first place, which doesn't happen for tanks or DDs (look for example a recent thread where a RDM wrote about getting kicked for refusing to AoE - no one defended them (at the point when I read the thread anyhow, haven't checked it since then)).