Im still working with my Staff getting use to all this and this same topic came up from him. It would be simpler and more efficient if the Preview Post button was with the standard one Post Reply. Virtually every other forum software has it that way so people are use to looking for it.
But, when you talk about the average user, I wonder what percentage of those average users will in fact use a [Preview] button in the first place? In fact, I (admin) almost never ever use [Preview] myself, since forumpackages like XenForo come equipped with a great wysiwyg-editor, diminishing the need of any preview for me. In the good old vBulletin 3.8 I alwyas used [Advanced options] or whatever it was called to post and almost never used Quick Reply. In XenForo however I find myself always using the Quick Reply. I didn't need a [Preview] once.
The reason there's a "More Options" button and not just a Preview button is so that the transition from member to moderator/admin isn't that different, I'd assume. Otherwise you'd be used to seeing the "Preview" button, then wonder where it went when you see the More Options button. Obviously if you're familiar to the software you'd know, but it makes sense in a way.