Works great until your kid's the one that can't join their friends on, say, grabbing dinner after band practice, because they have no way of telling you "hey, change of plans".

If your kid can only participate in things that are planned well in advance, your kid is going to be missing out on ~80% of gatherings. Because everyone else is in the habit of making spontaneous plans, made possible by interconnectivity.

Are we talking about 8 year olds, or 15 year olds?

I think it's fine to give your 8th grader a flip phone. A third grader isn't "grabbing dinner after band practice".

For sports practice, I'd just take the sports bus home; the 30-60 minutes between the end of practice and the time the bus left was perfect for a little quiet reading or homework.

For band practice, I'd call my parents from the office phone, or plan to get a ride home from an older student who lived nearby, or just accept that I might miss out on something when mom picked me up at 6:30 and that's ok.