I'm a big fan of subwoofers. Whether it's a surround or stereo, subwoofers will get you the bottom two octaves of the music far more cost-effectively and in a room-friendly way better than any "full-range" speaker I've ever seen.
All my room-based systems use stereo right/left subwoofers. I'll put my pair of JL Audio F-112 subwoofers up against any single sub, regardless of size of price. I even use stereo subs with my Dunlavy SC-VI speakers, which each sport a pair of 15" drivers.
The reason I use stereo subs is simple - they deliver more convincing and directional bass. But bass isn't directional below 80 Hz (the THX standard crossover point). True, but since most subwoofers use a gradual roll-off, be it 12, 18, or 24 dB per octave, it means that a subwoofer still puts out some directional information.
Even two octaves up from 80 Hz with a 24 dB crossover slope (320 Hz) a subwoofer still has some midrange output. Sure it's not much level (being 48 dB down), but it is enough to be audible in some setups and with some sources. The leading edge of bass transient is the primary problem. This leading edge contains upper harmonics that are will into the midrange. Mono subs can "pull" a low frequency instrument's image towards the center of the soundstage because its leading edge is coming from a more centralized location than it should because of the subwoofer. Stereo subs eliminate this problem.
Stereo subwoofers also "share the love" so that each one only has to work half as hard to produce a given output level. Personally I'd rather have two moderately priced subwoofers in a system than one that costs three times as much. Not only will the two have lower distortion at higher SPLS because each one will be operating in a more linear fashion, but with two subs you can reduce room resonance peaks and place them in locations with less room gain because you won't need as much room gain. And although room gain is a cheap way to get more output from a subwoofer, it is not without sonic trade-offs. The principal trade-off is that room gain is not linear - it amplifies only those frequencies that engage a room's resonance nodes. The result is often "lumpy bass" which while impressive, is not correct.
So the next time you begin to lust for those big full-range speakers in your dealer's showroom, perhaps trying a couple subwoofers with more modestly sized speakers might be a more cost-effective way to get better sound.