i think a lot of people go with GI Joe because:
1. they dont have maya 4.5 (i dont, thats why i use GI)
2. they see other ppl using GI Joe, like you said
3. and thats all i can think of

mental ray is also weird because its not part of maya if you get what i mean, its an external program called in by maya, whereas GI Joe is an actual mel script