The show runners needed to keep 11 separate from the rest of the characters so that her reappearance in the next-to-last episode would have have greater impact. It would have been dumb to have her just sitting in the cabin in the woods for nearly the whole show, so she needed something to do - because if you've got a character sitting around and doing nothing, the audience is sitting there thinking "Why isn't 11 here? Is she just watching tv?" But, if you've established that she's gone out of town to hang out with a gang from the Warriors, she is narratively very firmly divided from the other characters and "can't" just show up and give all the demodogs aneurysms. Then, when she shows up its a bigger moment.
Further, there's some Joseph Campbell hero's journey reasoning that says she can't get more powerful hanging out watching soap operas and eating tv dinners.
Plus, There's the episode-to-episode dramatic arc - Ep 6 ends with the demodogs climbing up the well shaft so you've got ***BIG DRAMATIC EXCITEMENT POTENTIAL*** The best way to heighten this is to make the audience wait and show them something that they are *ideally* just as interested in and that they've been waiting for - "Where's 11 during all this?" It sort of allows you to kill two birds with one stone.
Finally, I think we can probably assume they've got something lined up for 8 the future seasons, even if it's only dying dramatically to show that there's a big threat out there somewhere.