I don't want to come off as an argumentative ba@#@rd, but I just want to reiterate that questioning motives is not equal to a personal attack. Now perhaps we sometimes call an apple an orange, and maybe I'm not talking about the same thing everyone else is, but a motive is a goal or circumstance that motivates one to action. Any time you say "why would you want to do that?" or "what do you hope to accomplish?" you're questioning someone's motives. None of that is remotely the same as questioning someone's integrity or attacking them personally.
Here's a hypothetical example of where questioning motives is a perfectly reasonable response:
Post#1: How do you switch bluegrass fairways over to fescue fairways?
Respondent#1: Well, you've got to kill the grass with roundup.....(imagine extremely long and informative explanation here)
Respondent#2: Uh, why would you want to do that?
Original Poster: My course's fairways are not firm and fast and all of my favorite courses (which are firm and fast) have fescue fairways.
Respondent#2: Oooohh, if all you really want is F&F then...(imagine extremely long and informative explanation here)
Now, with all that said, I DO NOT condone personal attacks, questioning of integrity, or general suspiciousness and meanness toward our fellow posters. AND there are many times when motives do not matter, including George's example of the Oakmont thread (anyone who would get after you about that, George, needs a swift kick in the pants) and needn't be brought up. But if questioning motives is off limits, then we may have a great deal of trouble finding and answering the real question.