As a musician and composer, I have to say: I don't think a theme song being "humable" is a very good metric when determining how effective a soundtrack is. By all definition, a soundtrack's job is to compliment, not overburden. In the retro days of memorable, catchy tunes, game soundtracks were the most prominent sound effect - nowadays, aural atmosphere is achieved by so many disciplines that the soundtrack's prominence would act against it.
Orchestrations have produced many memorable game soundtracks, don't get me wrong. I think Halo Reach's is actually the best Halo soundtrack, Super Mario Galaxy has twice provided exhilarating scores, and games like Asura's Wrath and Rayman Origins have provided a uniqueness in full orchestral scores quite unlike their peers.
Not every game needs an orchestral score, this much is true. And simpler is often better, this too is true. But orchestral soundtracks have provided some of the best music in videogames, and I don't think I'm alone in thinking that.