I taught 10-12 year-olds for 9 years, and in that time, my opinion about this changed. I originally got quite annoyed when i could see that parents had done a lot of the work, and I even went as far as banning kids from taking work home.
However, my thoughts are that school work, and homework, are for the kids to learn stuff. (See my amazing educational philosophy?)
If a parent helping a child write a title on their posterboard helps the child learn how to do that for next time, then great. That's the kind of one-on-one help that I don't have time for in the classroom, and that some kids lack talent for, and need extra help with. Same goes for learning spelling words or multiplication tables, or writing sentences or whatever.
If the parent is doing that every time, then their child isn't learning that skill, and honestly, that's the parent's and the kid's problem in the long run, not mine.
Grades in primary school are for the purpose of letting parents know how kids are going, ultimately. If the parents are doing all the work for their kids on homework or assignments, then they already know how the kids are going, and the grades are pointless anyway.
I'm now tutoring a secondary student in English. I help him structure his assignments, suggest sentence re-writes, and proofread. I help less and less with each assignment, though, because through the discussions we have about why things need to be structured in a certain way, or why this word is better than that word, he's learning to do it himself. I consider that assistance an extension of what his teacher is doing, but doesn't have time for during class.
I see no point in letting kids struggle through assignments they don't understand, when some assistance could help them learn the process for next time. Just doing the whole thing every time, however, does the child a disservice.