there is nothing wrong with the tautline,and it should still be taught as basic technique. It is easy to tie and untie while working and is a good hitch to be comfortable with if working without a splittail. It has limitations which must be respected, such as its tendency to creep.
IMO the blake's is a more reliable hitch, not as easy to tie and untie repeatedly( could be debated) , but an excellent all around hitch to use with the split tail system, which allows one to move more freely in the tree.
Next is a blake's with a slacktender
Then french prussik
lockjack next(if your a gearhead that's got the money)
I think being trained to climb with the most basic set-up,i.e. no splittail and a tautline, teaches invaluable skills, such as good routing technique in a tree.
If you have to flip-in and untie/retie everything everytime you reset your line or have to bypass a branch, then your routing skills will improve.
That being said i don't personally know many production climbers that still use a tautline but I think it's a great idea for training. I was taught with a blake's split tail system, and i think there where some concepts that i may have "gotten" sooner if i were trained without a splittail, even if just for a short while.