Exactly. A +6dB peak @ 15 kHz is hardly a concern in real life.
There is a good reason the "listening window" is from 200Hz-10kHz, not 15kHz.
Well, aesthetics is pure preference. A lot of people find big towers much better looking than little bookshelf speakers and subs. And placing the subs in their best spots could mean in the middle of the room or behind the couch or some odd places. Not very good for aesthetics or wife acceptance.
And having 2 built-in subs on both sides of the room would smooth out the bass response. Most people place one sub on each side of the their rooms anyways. If the best placement is in the middle of the room or behind their couch, most people would still place their subs on each side of the their rooms. It may be a slight "compromise", but most people would rather do that for the sake of aesthetic, room limitation, wife acceptance, etc.
I love the idea of having towers with built-in subs, but only if the amp is external -- like the RBH SX-8300 or SX-T2.