Yes indeed. I could take classes at night in between hospital shifts, and go to school part-time. The reason I'm choosing to spend WAYYY too much money on a fancy structured postbac, rather than continuing to live my office-cubicle life (where the bills are paid and the first round's on me!), include:
* Structure. A postbac means I'll have an advisor, a class schedule, and a plan to complete the thing within 18 months.
* Resources and support. I'm not naturally great at studying, and haven't had a science class since 9th grade. A postbac means I'll have tutors, faculty hours, MCAT prep, and a corner of a library that can be my "office" for 8 or 10 hours a day, if I need it.
* Legitimacy and esteem. My transcript reveals years of doing pretty well, considering all the other crap going on in my life at the time. It doesn't show a 110% full-time course load, and it doesn't show academic brilliance. The quality of my work is extremely varied, if grades are the only measure used.
I'm clearly capable of brilliance, if you look at the details, but med school admissions committees need us to make it easier for them to spot the good candidates. For me, the postbac is a machine for turning the 2004 version of me (good intentions, talent, potential) into the 2006 version of me (all the above, plus proven track record). I happen to think it's a better machine than one I could build for myself. Your mileage may vary.
Good luck to all of us, DIY or 'official.'