A'ight. Well, I have a MBox, so I can only ever do two at a time anyway. I also doubt it would be cheaper to buy four or six good mic pre's than to buy a used Mackie.
If I need to record more than 2 tracks at a time, I either rent bigger hardware or go to a studio. I don't do it frequently enough to justify an 002 or similar. Even if I did, I'd probably get a 1604 or similar (i.e. something with channel outs) to put in front of it.
I'm also of the belief (and I know you generally don't agree, and that's fine) that the signal hitting the computer hardware should be as ideal as possible. For this reason, I like to have some gain and EQ controls in front of the MBox, especially when I'm recording a voice or an acoustic instrument. I like to blend two mics into one track in some cases, and that's where a mixer comes in very handy.
Lastly, I appreciate the convenience of having all my instruments wired up to the mixer, thus allowing me to quickly send something to the monitors or to "tape" without f-ing around with patch cables. Its a minor consideration, but anything that makes my recording process more pleasant is always good.