It's just not a good idea for these poor servers to be answering this question 100s of times.
We are talking about dedicated servers here. Not an on old pentium running on a 56K connection. If a server cannot hold several queries (something less than a 1kb), then it will not be able to support more than a few players.
On Minecraft there are thousand servers (I'm tracking myself more than 8,400 on my Minecraft servers list) with more than 100 players connected along the day on each server. Theses servers are registered on several lists and are queried from every direction several times per seconds: and there is no problem.
Your system reminds me the one using by Counter-Strike. But it was using two ways:
- a first way with a master list like your idea
- a second way with a basic query protocol (to allow users create their own script to display server status on a website without overloading the master list)