Agreed- it's as boring as mud. They should have just had Theon completely disappear for a season- THAT would have been interesting.
He would've had to disappear for at least two seasons then. After that much time you don't even know whether you'll get the actor back. That's a risk I don't want them to take on someone who's as good as Alfie Allen (who's also a main cast member).
There's something called a holding contract to cover such situations. A show can release a performer for a given period but require him/her to return when needed. It's how Emilie de Ravin was excluded from the fifth season of Lost
without any risk of her moving elsewhere.
I don't mind Theon's torture being shown onscreen, but as with several of this year's other expanded storylines there's no reason for it to take up so much time. The first four episodes worked because of the reveal of the boy's duplicity (and book readers should remember his identity still hasn't been revealed to, and constitutes a spoiler for, the TV-only audience), but the stuff in "The Climb" and this week's episode doesn't add anything except more over-elaborate TV-psychopath mindf***ing. If the show's writers were more talented, they could dramatize Theon's gradual psychological decay, but instead it's just "torture of the week."