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: 
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.
I think TAing helps in evolving a graduate student's character. Teaches him/her how to plan courses, interact with students and enhance public speaking/presentation skills. Its the another road "step" into academia (other than research). Perhaps that why it is necessary in many schools even for those appointed with full research funding. 

What I don't like about the current system is that TAs tend to teach the same course over and over again! It gets boring and many loses motivation. Also, there can be many grey and shady areas between professors and TAs/graders. For instance, professors asking TAs to do the bulk of the work, being available for extra office hours, write exams, update notes etc. In such cases, TAs tend to agree to these things "most can't even say No!".