I graduated college with a decent GPA, but I hadn't taken all of the prereqs. I chose to take postbac classes (I was NOT in a formal program) for a number of reasons.
- GPA. Although my GPA was fine, it wasn't great. Postbac classes figure into your undergrad GPA, so I was able to raise it with a minimum of muss and fuss.
- Grade inflation is considered (rightly or wrongly) to be a given in grad programs, so even great grades in a masters degree often can't make up for a lackluster undergrad transcript.
- Time. I busted my tail, but I got everything I needed to done in a year, while working full time.
- Money, money, money. Since I didn't have to quit working, I didn't have to go into debt or anything while I was taking my classes. Also, I only had to take (and pay for) classes that I needed, not an entire program.
- From my conversations with adcom members, I believe that having a masters degree just doesn't help you that much in admissions. Some people may disagree, but that is what I was told when I was considering my options.
Good luck!