Questioning the OPINION of a moderator, or anyone, I don't think is an issue. However, in order to be taken seriously for debate, the questioning itself shouldn't be inflammatory, accusatory, or anything "rude." It can be difficult to get words and feelings across the way they were intended when using a text medium. Along that line, in general people do not always take criticism well.
For example, I have seen several posts questioning where the information has come from, like Ly's extensive diet research. I'm sure that being a moderator here might get frustrating -- having to answer the same questions over and over again. I know I wouldn't have the patience for it!! There's a search feature for a reason, you know?!
The point is that when a moderator or experienced member gives advice, or warnings, they do so with facts to back them up. If it's a subjective opinion on something non-piggies related, then it is my belief that they are entitled to their opinion, and if anyone wishes to question their opinion, the proper question would be, "Why do you think that way?" Most people have based opinions on facts or conjecture, and asking for the reasoning behind their opinions is SUPPOSED to lead to a learning experience.
And I always believe that heated debates, definitely belong in The Kitchen section.