Its usually the really bad teachers who need help. Its not an honor, its a necessity if your child is in a grade where the teacher can't handle the class.
A competent teacher doesn't need or want assistance.
Absolutely not true. The great, well organized, confident teachers allow parents in the classroom! I volunteer weekly in DS' class and I do a variety of tasks--administrative (copying, cutting, laminating, etc.) and class related (helping with centers, taking a reading group, testing math facts, etc.). I wish DS' teacher had more time with the kids and I am happy to help with the busy work. I do think teachers have a huge job with not enough resources and I am willing to help out in any way I can.