At the moment, I can think of 3 good reasons to go the PT route. There could easily be others, but these stick out the most to me.
1) To get into a school you couldn't get into the FT program for. Think Georgetown, Fordham, maybe some others.
If this is the case, your employment options should be wide open. You can spend the school year doing internships, working in law firms, working in a legal department, working as a patent agent (for the geeks out there), etc. Summers will still be your summers, so you should be at no disadvantage there, although you will be a year behind the students you start with (i.e. you probably won't get a class rank until you have completed 2 years of PT work). Also, if you're using PT as a back door into a program, it is likely that you would try to switch to FT so you can finish in 3 years. This would obviously nix any work during the school year and may provide difficulty obtain summer work both summers - not having a class rank at OCI could be a little troubling depending on the employer.
2) Because you have to.
You have dependents that need your income, and you can't/don't want to survive on loans. You will likely need to keep your main job throughout school, so your options will be much more limited. One suggestion I have heard is to quit your job the last summer, get some cash working as an SA at a firm your last summer, and then scrounge around for whatever you could get to make it through that last year. However, this suggestion came when law firms were actually hiring.

This is generally the toughest situation. A reduced load at work would help. You could also look into volunteer opportunities on the weekend, but this is a tough path in general. Some people take leaves of absence to go work/volunteer/go on trips to Europe during the summer. Some have the vacation to cover this.
3) Because it makes financial sense.
Some people can afford to pay for law school as they go due to their incomes. This might not be bad compared to taking out loans to cover cost of living, tuition, etc. At the very least, working should prevent needing to take out loans for cost of living. This would also make sense if you're making really good money and desire to be making very little when you come out (such as if you are making $150k a year but deeply want to be a PD). So working can be seen as a way to prevent taking out as much in loans. Not the worst option in this economy. But then if you keep your day job, you're essentially dealing the problems faced by #2. If you're just working to cover cost of living for yourself, you might be able to take on a less demanding job and pick up legal work on the side.
Most people I know in my PT program that could go FT switched to FT after the first year. Even though there could be some advantages for people who can pick up more internships/externships/legal employment during the year while going PT, most people would much rather just finish school and not have to deal with the tighter schedule of a PT student who works during the day.