Just to clear things up (without getting back into this mess, Kaelyn and Bob are doing admirably in my stead):
Druidic feats from Eberron do not represent membership. Eberron features something like five major sects of druids in a region called the Eldeen Reaches. For example, one sect is called the Children of Winter, and it has a feat associated with it for druids that belong to it called Child of Winter. There is also a prestige class called Eldeen Ranger for rangers who belong to one of these sects. Neither the feat nor the prestige class is required to be a member, nor does the feat include any particular drawback.
Is this argument really all about that one feat Manic-Depressive? If the Bleakers knew how much was being made about one feat, they'd probably weep, if they could be moved to care at all.
It really gives some mixed bonuses and minuses. Just think of it like Power Attack. You get some advantage, but in order to do so, you willingly accept a penalty. The difference is that this one doesn't turn on and off.
Your argument appears to distill to 'Planewalker should not be allowed to be creative.'
So, to be a member of the Harpers, you have to take a level in one of their prestige classes?
I present to you your own frequent admonishment: Read carefully what I wrote. To gain any mechanical benefit for being a Harper, yes, you must take a level in one of the prestige classes. No feats give you Harper abilities, and certainly no templates.
I haven't read the Eberron book, so I was unaware they had such feats. Assuming they're there (why would you arbitrarily lie to me, after all?), there does seem to be a bit of a precedent for feats as group membership. Well, gaining abilities for such, anyway. I suppose the part of my point that still stands is that we still don't have anything in 3.5 (no breaking precedent is allowed, of course) that represents powers from group membership as a template.
Pants of the North!