Believe it or not, I used lightwave for a little over a year before I discovered mouse menus. I guess its the fact that the average user will only really ever discover these by accident, and even the inclusion of the "mouse menus" in the "edit menus" don't really clue in users as to what they are referring to (they should should be labeled like "CTRL+Shift Left Mouse Menu" so people aren't guessing).
I know i've mentioned these in various other tutorials i've done... but alas, I feel users need to be aware and I don't consider it an obvious thing.