I'll tell you why not. Because it's a class, and classes focus on presenting content, not on making sure you learn it.
Getting the results you want on standardized tests like the SAT and ACT has consequences for your life. Therefore, it's important to find the support that will help you maximize your results--support that focuses on you, the student.
Test prep companies like Kaplan and Princeton Review have brand recognition, which appeals to the herd mentality. Just sign up for a class! There's probably a class in a school or an office not far from home. What else is there to think about?
Plenty. Have you stopped to consider . . .
1. The materials they use. Kaplan's materials are mediocre. They recycle and reuse the same practice tests year after year, even when these materials DO NOT APPROXIMATE THE ACTUAL TEST. Kaplan produces quality materials for higher-level tests, but their SAT materials are too easy and not on target. Princeton Review is also off-base: their materials tend to be too difficult, too oblique, and also NOT ON TARGET TO THE ACTUAL TEST.
2. The people who teach their classes. The instructors who work for these companies are usually young and relatively inexperienced; they're bright and have been trained in the company method, but very likely have NO EXPERIENCE in working with individual needs. Nor are they likely to be around for the long haul; their investment in your outcome is small.
3. It's a class. If you take a class, you will gain some basic information about the test. But with 25 people in the room, the instructor cannot devote time to help individuals overcome stumbling blocks. So you'll probably need a tutor anyway.
Kaplan and Princeton Review are not the only tutoring companies out there. Each year, more crop up because they employ relatively inexperienced instructors who are trained to deliver their material and not much else.
There is a big difference between presenting information and learning. A tutor should help you learn. Learning is a process of self-understanding: of gaining awareness of your blocks to reading, of seeing how you respond to an author's intent, of retraining yourself to avoid personal pitfalls. Your tutor's job is to facilitate this process; you'll never get that from a test prep class.
It's said that mastery of a skill or a discipline takes 10,000 hours of practice, or about 10 years. Isn't that what you want from your tutor?