My understanding is that Masters and PhD students often need to TA -- even if they're already research assistants to a professor.
I'm a first year undergraduate who took courses heavily TA-reliant last semester. My impression is that my TAs (oh they were helpful people) don't really get much out of TAing. The job just covers some of their costs and their tuition.
Sure, you may learn a bit about teaching others, but I doubt that you may learn much after three weeks of doing so. Many of the users on this site were once or are TAs. Do you agree with my view?
If TAing is really a necessary evil, are there better ways to fund your education?