I think having a single sub is a bad idea, and I'd prefer not even having (just) one. But having more than one sub makes a lot of sense. You aren't adding subs to increase "boom boom" bass; Quite the opposite - you are doing it to make the bass smooth and the transition seamless. If you can tell the subs are on, you don't have 'em setup right.
So, if I'm understanding correctly, if my budget could allow me to get one really great subwoofer or two pretty good subwoofers, you think I opt for the two instead of the one? Do you think it would be wiser to just save up some more for multiple higher-quality ones?