It's not our place to say whether people should spend time learning how to script. Perhaps they just want to create one script for one specific purpose. Don't forget that mIRC is first and foremost a chat client, it's not supposed to be a general purpose programming environment. From this perspective code completion would benefit your average "non-hardcore" user, and would also benefit more experienced people by saving them extra keystrokes.

I don't think the fact that it might or might not discourage people from looking in the help file is a very good reason not to include this feature.