PA coach would probably be a better source for what coaches want - I coach outside the more regular "school" ball. It's only at major events that we get a full halftime. When we do, I am so used to a pretty short halftime (3-5 minutes) that we are never late. I am only guilty of "maximizing" 30 sec TOs
My own thought is that most coaches probably have a feel for how long they have at haltime without ever checking a clock. If they overextend, they have to know they are doing it. So even though the notification is a requirement, i am willing to bet that it is not any surprise to those being notified. You really develop an internal clock and you know what you can get done in your allotted time. If you don't, you can't possibly make effective use of the time.
As a completely tangential side note, I do like having the full halftime. It gives you more time to talk as coaches before you go to the team, and allows a little more emphasis of each point you want to make so you aren't racing from point to point. But the short halftimes give you the good habit of not putting out too much info, because players can only focus on a couple of major things anyway. That's a good habit when you get more time.