Easiest explanation on the "crap" theory:
In particular for new machine, but perhaps old machine that were bought new as well...you paid a premium for your CPU (for example, let's say you bought a 1 gig p3)...add software modem, now you have a p3-700 when you are online, so you really paid several hundred $$ for that modem. Good deal eh? This doesn't even consider compatibility issues.
If you're lucky enough to have good phone lines or live next to your CO, you might actually get a decent speed, but again...at a major price. Granted, for the situation that the guy above was talking about (his folks), extra clock cycles are gonna be wasted anyway...but if that's your concern, you better install the rc5 cracker and seti@home software just so those extra cycles aren't "wasted" when your folks aren't online.