I’m not a huge fan of Eclipse either (I prefer jEdit for its simplicity), but I think one terrific reason to go with a java-based client for their project is cross-platform compatibility. I’m on a Mac here and wouldn’t be able to use EditPlus (or any other Windows only editors). Would you have been very happy had they chosen to go with TextMate (a Mac-only editor)? I think that Zend took the compromise on this one and decided to pick a cross-platform editor so that they could focus on getting the framework ready instead of worrying about catering to a single platform.

-Ian