IMHO NOT operator could be very useful with global search enabled, I have used it many times in eMule. For example try searching for "Alien" on aMule, you will get lots of "alien vs predator" making search results difficult to read, a simple "alien NOT predator" query solved the problem. In such cases I always prefer to search for files in my Windows machine then copy ed2k links and paste in aMule.
So please consider adding NOT operator too if it is not too difficult (I think that it is easyer then OR to implement).
Frankk