I don't think it's rude at all, but I can only speak from my own experiences. If I put time and effort into a long request through PMs, and all I get back is a 'No Thanks'? I could take the priviliged high-ground then, and consider that rude, for not even taking the time to elaborately sit down and think of how to convey their lack of interest? I wouldn't, obviously, since that is completely ridiculas. And, I honestly feel like it is, as well, to think we're owed an answer. We're all here to have a good time, and not all of us has a ton of time on our hands, some dropping by just to check on posts, both ic and ooc. On some of those days, I've let half-assed requests slip through the cracks, and thought nothing of it, just like many of my own requests has also been ignored.
None of us can have a good time, all of the time, if we always had the needless fear, of what other players here might think. Feeling sorry for a request, not answered, on either side of the case, is just not worth anyone's time, in my opinion. When I've been ignored, I've largely just moved on, happy with the good partners that did write back, and when I've ignored others, I am sure they've done the same. And, well, if not? Sorry about that, it was nothing personal. ^^