@1 Havn't had that problem really, I normally find people using classes instead of jobs are more of a hindrance than a help.
@2 I think those are just people playing their Job badly and not utilizing their skills? Like those NINs that only use Huton you run into.
@3 Dragoon, Ninja, Bard and WAR can all cross-class the 5% Mantra too. When it actually count's for something you will most likely have one of those classes in your party, and they can pop it when necessary instead of you having to lose out on job skills. E4E is ok, but both SCH/WHM are going to have that too and a third isn't going to make that big of a difference? A non-traited virus does not debuff int either
If you have a group of friends that is willing to work with a buffer class only then that's fine, I guess. But I don't think it has a place in regular Duty Finder play with randoms.