I think you are missing that a single-player campaign can be fun and entertaining to play through because of the gameplay even if the story sucks. If you ask me, that's the case with more than 90% of the games out there.
Sounds to me as if you've already made up your mind. If you have an itch, scratch it! I for one wouldn't want to play ME on consoles 'cause I think the controls are clunky as hell on the 360 ver which is sad 'cause I like the controller.
I've never been a big fan of the Halo games, it's probably the most overratted franchise out there in my mind. But I'm giving Reach the benefit of a doubt and will be picking it up day one tomorrow. So I say go for it!