A few months ago I got OB on PC, so that I could try out some mods. I must admit that I've always felt that OB is a bit empty and could do with some extra content. Nevermind graphic mods, they're just for aesthetics, it's adding new content that really enhances a game and of course fixing bugs etc. But I don't feel the same when it comes to FO3, for me FO3 certainly doesn't feel empty and feels a lot more fleshed out, yes there are some mods that would add to the experience but modding to the same extent as OB really doesn't seem necessary, e.g. Mart's Monster Mod in OB is a must have mod in my opinion and really brings the game to life but I just don't feel that Mart's Mutant Mod is a must for FO3, the world is populated enough ([censored] Rad Scorpions).
Skyrim has a lot more content than OB, from an extensive crafting sytem, to more wild life and even different types of fish. All things that mods added to OB and I bet there's more enemies to fight too, not to mention Dragons flying overhead. Of course there will be some good mods for the game but I just don't feel they're as necessary compared to OB. But we'll all find out in a few weeks, if my hypothesis is correct.
It does sadden me to think that some people will add mods, long before even finishing the first playthrough but that's their choice, some people just love mods.
I should point out that I don't think OB needs mods, just that they can enhace it, more so than FO3 and Skyrim.