IMO, there is no major benefit to a fifth year for people planning to go into public accounting. The vast majority of campus hires only have a 4 year degree, so it doesn't give any extra advantage there. Once you're hired, at least at my firm, no preference is given to people with a masters degree vs. bachelors. All promotion and compensation decisions are based on how you actually perform while working, not your credentials. I've sat on my office's annual review committee for associates, and who had an advanced degree and who didn't was not a subject of discussion. My pessimistic side says that people do it because the schools market it as a great way to get the 150 credits (and pay a 5th year of tuition), and there is really no competing voice suggesting it may not be the best idea. Even when firms recruit on campus, they'll tell students who are interested in a 5 year program that it's just as possible to get a job after 5 years (which is true). The people recruiting aren't going to get into a discussion of positives and negatives because a) they're there to recruit not be career counselors, and b) they're not going to be openly critical of a school's program and possibly jeopardize their relationship with the school. My less pessimistic side says people do it because they think it'll be easier to do the classes before they start working, which I'm sure it is but the firms are also pretty flexible in terms of allowing people to leave work for class so again it's definitely do-able to take classes while working.
In terms of the requirements for the 150, it varies state by state. In general, the states will have a minimum set of required credits in specific courses (e.g. 24 credits in accounting, 6 credits in finance, etc), and an overall total requirement of 150. Meaning if you are an accounting major and have completed all the specific requirements during your 4 years, the extra credits to get to 150 can be in any subject you want. It doesn't even have to be business related, you could pick up a masters in fine arts and it would still count. Your state CPA society should be able to help you understand the specific requirements in your state if your school isn't helpful, and I believe the AICPA website for students has info on the requirements as well.