Question: 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?

Answer: 
It may be necessary; some universities require students to TA even if they have other sources of funding. It need not be evil. It depends on your attitude (do you want to help students learn?) and which faculty you are working with (Do they care if you help students? Do they care if you learn something yourself?). Unfortunately you usually do not get to chose who you work with. TAs (and also faculty) should receive training in how to teach, but may not.

There are many other ways to fund education, some of which pay better than being a TA, and some of which are more prestigious. They all have limited supply.
Sometimes? Here's my notion of when it's evil: When a department admits a ton of graduate students it has no particular inclination to mentor much less graduate because without a steady stream of TAs it can't keep its undergrad classrooms staffed. Been there, did that, burned out.

For Ph.D candidates intending to teach, TAships are not intrinsically evil; well-run (not a guarantee, sadly), they are straight-up job training.