I think parent volunteering is only a good thing if the teacher wants it and the parent is good at it. Our school encourages volunteering and each teacher has a sign-up sheet of shifts they need covered. If you can't make your regular shift, you're supposed to find another parent to replace you. I have mixed feelings on this. Some parents do really well with the kids. Others don't. My son has brought home work before that was wrong and he said a parent helped him with it. Some people sign up for shifts that suit their schedules but not necessarily their strengths. For example, I would never sign up for a math center. Math is not my strength and I'm not good at explaining it to others. I try to sign up for spelling, reading or writing since those are my strengths. A few years ago, I did a weekly spelling center with 2nd and 3rd graders. The kids did different activities that included defining the words, finding the suffix or prefix, finding the root word, writing sentences, etc. Another mom also worked during this time because she could only work in the afternoons. The problem was, English wasn't her first language. We would sit outside and read our assignments until the teacher called us in. I would have to go over the spelling list and the activity instructions with the other mom to make sure she understood what to do. She often hadn't heard of some of the words on the list. When my son worked with her, he said he didn't understand some of what she said and she wasn't much help to the kids.
This was a case of a mom needing to sign up for something that fit into her schedule, not necessarily her strength. Working in the classroom wasn't mandatory. You could help with fundraisers, gardening, parking lot duty, etc. I figured it was up to the teacher to ask this mom to change assignments but she never did.