Excellent question and unanswered here......especially when the choices are a good bookshelf vs a much larger floor model from the same maker which is much more expensive, where the chief benefit is lower bass.....of course 2 channel purists might want to waive a subwoofer, but for most, why pay for quality bass that is bypassed by the 80HZ cutoff going to a sub....
Please remember, x-overs are not brick walls. I think I have the impression that most receivers have x-over at only 12/db per octave. Some fancier pre's might have 24db/8ve, and I suppose some other devices do 48db/8ve (or perhaps even steeper).
I believe I've even read that many receivers do 12db slope on the sub, and only 6db on the speaker outputs, as its assuming some natural roll-off already in the speaker.
IMO, the midbass might be the ONLY area where a decent HT suffers compared to the theater. That's my impression, even with my HT vs cinema. I use tower mains, xover'd 80hz. As for PQ, SQ, LFE, immersion, and comfort, my HT wins.
Also,
even if you had speakers that were truly full-range, room acoustic issues may have the x-over as ideal anyways. I think this probably is the situation most of the time.
-jostenmeat