I find comments helpful in the beginning stages of a map. Its quick and easy to cycle through a long list looking for the section you need. Its also helpful to be able to do this when all systems are still being updated. The stage my map is in now, nothing but the strings need work, only storyline, so all systems are in place. Comments have long since been deleted.

Only have comments around major things, you don't need to comment each trigger, just for a group of triggers, explain them a bit with a blank trigger at the top of them, there is no need for excessive commenting, as it makes it annoying to edit triggers since you can't see their actions and conditions to start with.