Fine: we will all switch to TS. The point is, I still hit you 500 damage, making the meta game (transformation stalking/running) relevant. I'm all for this!
Why TA is still a possibility: coin sets provide additional HP -- with TA, the extra damage brings the base hit ratio nearer to that of a coin-less game; however, it is slightly longer, adding to the intensity of the match. No one enjoys short, easy matches, especially myself. If you use TD in PvP (TD is acceptable in BvB because it has a weakness [unable to damage the base efficiently compared to a player with TA]) you're making a statement. This statement being you are too noob to handle transformations (running or stalking). As I stated previously, I'm fine with using TS, as I will still beat your ass in transformed play.
HOWEVER:
Why TA is used over TS: using TA actually enables sloppy trans usage to pay for flaws. If you suck terribly at utilizing trans, I'm going to hit you 1600-2k damage by the end of your duration. This enables an additional skill requirement (proper transformation usage). You see, Lyric, at the top tier, I like to make the game play as skilled as possible. You're simply dulling PvP down to a terribly newbish level by using TD and avoiding your pathetic weakness (the meta game). Additionally, the transformation gains a large benefit if used effectively (500+ damage per jump attack if used with efficacy). I can't be hit regularly in transformation (you'd be lucky to land a single hit on me throughout its duration). So this means, "OMFG TA GIVES 300 DAMAGE", is negated. If a good enough player is stalking you in transformation, running will be your only option. TA simply separates those who adhere to the epitome of play within PvP from all those other newbish idiots.
TA is where the pros sit my friend. Forever and always.
TL;DR -- TA enables the most skilled aspects of the game to be brought to fruition throughout game-play.