The question is simple:

If you have two channel system with floor standing speakers and listening to music only, would you integrate a sub into your system or not?

My answer would be a definite yes. Even if main speakers are capable of producing bass frequencys, it lack advanatges of having a dedicated sub. Such as ideal subwoofer location for best bass frequency responce, and more "sonic" control over those frequency range.

Not to mention less stress on the main amplifier if a subwoofer take care of the bass