There are too many cheaters playing Minetest. Run or fly for a few minutes to some remote location just to be followed all the time by other players with much higher speed. No way to hide, no chance to get away. Just hope they don't steal. I saw other people build complex objects in no time without even moving.
You can't compete with people like that. As long as the game is about building this is only a bit annoying. But if there are any levels, achievements or other competitions this won't be fair at all.
This game is open source which lets people modify clients with ease, you can't stop this - if a game lets access to it's source, there will be mods, hacks, you tell!
@Evrim Of course, the server could check if a player moves too fast (and actually the server could control all player movement, while currently it's all client-side), the server could check if a player placed or removes blocks too fast, and so on. It just isn't done currently.
I have an idea, implement an encrypted file that can be downloaded to, let's say, "/keys". A server can require this file from the users. The file will also check to make sure that the client is in accordance with specified options.
I'm against serverside movement.
However, some movement checks could be added to prevent players flying/moving too fast (see NoCheat, which is a Bukkit plugin for Minecraft - it does its job greatly. Prevents flying, mining/building without swinging arm, moving too fast, etc...)