You'd be surprised by how many players don't have the maps. Hopefully 343i will update playlists as we get closer to Halo 4's release to require DLC for more playlists.
By the way, I was involved in a lot of the testing for these spooky variants. That's how I heard.
Spooky House was for camping, and while it was cool at first, I'm glad 343i decided to promote teamwork in LD rather camping.
Of course the maps are not original. They're infected VARIANTS. That's what 343i, and the H:R community wanted. I'm not sure how involved you are with the forge community, but there is a lot more to it than "a few crates and clolour effect." We did not just randomly throw junk around. It's all for a reason, whether it was to block lines of sight, manipulate traffic or un-camp a position. Not to mention the amount of testing these maps have undergone. Yes, these maps were TESTED before they were even looked at by 343i.
As far as original FW content goes, yes, there are a lot of good maps out there. The problem with that is that they would have to go through much more strenuous testing than a variant of a vanilla map.