Sure, I agree with that. That's why it's so painful for the kids who either aren't allowed or supported by their families to participate. Every school I've been in does tons of things that celebrate fall and Halloween, removing the 2 hour time suck of a costume parade for a small amount of parents and having an after school event doesn't change that. (Plus the added thing that some kids are excluded that I talked of earlier.) Parents want it all, so teachers end up skipping some of the cool educational opportunities the holiday provides to have the kids parade through or around an empty school (all the kids are parading) in their costumes. Missing art projects, math games, science experiments, etc. to do a Halloween costume parade just doesn't make sense to most teachers, who usually vote against the Halloween parade - at least in my experience. People just like to blame "religion" for things. IMO the best way to "have it all" is to have the after school events or have people do the costume thing on their own after school.
At the school I was at the teacher's overwhelmingly shot down the costume parade. However, we have all sorts of fun. Some of the older classes put on plays, one teacher knows how to milk the silk out of spiders and is in great demand to demonstrate. Lower grades usually do cooking and/or candy math games. Everyone does art projects. All of those things can be adjusted around whomever is in your class. Then we have a carnival after school. Yet, every year we hear "they're cancelling Halloween due to religion." What?