Generally speaking, playing “3.PF” is extremely common, and initially, at least, Paizo touted “backwards compatibility” as a major feature of Pathfinder – that you could play 3.PF.
Now, the backwards compatibility isn’t as complete as it might be (they didn’t simply write new 3.5 feats/classes/spells/etc., they rewrote a number of the base rules and thus conversion is required), but it’s still nominally a feature, and in a lot of cases does more-or-less work out.
The issue ends up being that because the two are “close enough” for a lot of groups, they don’t explicitly define which way everything is going. Certain, “big” items, those get might get definition – “Pathfinder feat progression, but use the 3.5 version of feats found in both,” for example, is something I have seen at a number of tables.
But stuff that tables don’t pay much attention to until the question comes up? Things that we actually see questions about here? Those things are not defined ahead of time. Indeed, querents typically would not even know that the two differ on the asked-after mechanics.
It then becomes the answerer’s responsibility to address the conflict: to describe what each game states, with references as appropriate. It becomes the community’s responsibility to recognize the question’s requirements, and thus upvote only those answers that address both, and downvote those that address only one and ignore the contradiction between the two. The only good answers to such questions are the ones that get the details right for both. I strongly believe it is the community’s responsibility to downvote an answer that gets one right but the other wrong – with a clarifying comment pointing out the problem. It is also everyone’s responsibility to come back to the answer and un-downvote, and perhaps upvote as appropriate, after the answer is edited.
This makes such questions, when they touch on areas where 3.5 and Pathfinder have disagreements, somewhat more difficult to answer, but far from being too much to expect of this community.