BigWill has a good point. The bass from a sub isn't the same as from good towers. In fact, it's not even close in a large room. (My experience is comparing the M22ti to the M60ti in a large room with a 10" subwoofer crossed over appropriately for each speaker). The bass from the M60s is much tighter, and gives much more presence and dynamic range than bookshelf speakers + a sub could ever hope to reproduce in a large space.
That being said, room size and budget are big considerations. If you're in a small to moderately-sized room and/or on a strict budget, an excellent bookshelf (such as the M22s) + a good sub may be more appropriate. If you're in a large room and you already have a sub, then the floor standing speakers are certainly the way to go if you are willing to pay the price differential.
In short, it's not a one-size-fits-all solution, but depends on your room size and budget.
