If a question has really atrocious grammar or spelling, I will automatically expect a throw-away question, because it is probable that the author, if they have not taken the time to type the question properly, has not taken the time to think it through properly either. Grammar rules are there to give logical structure to an argument. The rational, ‘conscious’ part of the brain thinks, with few exceptions, in words. Thus, a logical thought should already be formulated with decent grammar, because that is how a person can check that it is, in fact, sound and complete; and so if the written expression of that thought is written with very poor grammar, the author was either extremely hurried, or the thought was never fully-formed in the first place. To bring this back to the original question, this principle is not without exception, so I will read something with poor grammar, but I will have that first impression in the forefront of my mind, and it will affect at some level how ready I am to accept the author’s argument.
An additional angle is that, in a place such as Fluther where intelligent debate is respected, bad grammar is a sign that the author has not taken the time to observe the community, to get to know it and its conventions, and is simply there to get a quick answer or to fulfill an agenda. So I will be inclined against them for that reason as well.
So far I have assumed that we are talking about online written discourse. If I am talking to someone in person, or am hearing them talk, the first part of my argument theoretically applies, but here I must admit to more prejudice than it can justify. Someone who speaks with bad grammar generally comes from an entirely different culture than my own, even if we are native to the same city, and I am still working on breaking down my instinctive snobbery and aversion to that difference. I can override that initial impression with relative ease, but it will be awhile before I come to the point where I don’t feel it at all.