here's my opinion, but take it with a tad of salt. There are two schools of reasoning for this question. 1. profs have had experience with years and years of students. since they've seen so many, they should be able to accurately predict how you "stack up" to the group per se. A TA is a young grad student usually. no experience teaching. probably less objective. not really someone who can attest to your potential. 2. you want to find someone who knows you and can write a personal letter that supports your candidacy. so, the reasoning is you should ask a TA who knows you well over some aloof professor who you never spoke to.
So, in my opinion i think TA recs aren't quite optimal quality. But, if you have two good recs from faculty who support your strengths and qualities, then i think adding a 3rd from someone who knows you very well personally may even be better than antoher prof who says "student x got grade y in my z class, s/he was very smart. blah blah" do you see my point. Recommendatiosn are to give adcomms the big picture of who you are. you need to be smart, nice, talented, good pesronality, etc etc. get your recs to touch all aspects of your life and paint this pic of you. so, i dont think a TA is necessarily bad. but, i'm just a premed. hehe. good luck peeps and stay sane in this process.