[kkylin]

Not to say that these are not issues, but the treatment of students is also highly field-dependent. In addition, in fields like mathematics (mine), most postdoctoral positions (even very good ones) come with a certain amount of teaching, and students who have not had significant classroom experience sometimes will have difficulty finding good positions.

Also, how much TAing one does really varies a huge amount between fields. AFAIK students do much more teaching in the humanities than in STEM, and within STEM departments that teach lots of service courses (e.g., math) will tend to have more TAships than departments that do not.