Kayle is my most played champion over all. Shyvana was my most played champion last season. I've tried every combination of runes and items imaginable on those champions, including the LT rune. PTA is just way superior to LT on both of them.
LT has the potential to be great on Kayle but it's wind up time makes its a huge pain in the ass to use effectively in the early game. PTA makes her early lane phase so much more threatening.
As for Shyvana, she has an auto attack reset and her Q counts as two autos. PTA gives her an almost unfair amount of burst damage, especially after she finishes BloodRazor. It's so good on her that it's straight up stupid not to take PTA on her. PTA feels like it was made for Shyvana. Her entire kit synergies perfectly with it. Also, I would never build AP on Shyvana. I'd simply learn an AP champion instead. I've never been a fan of those kinds of off meta builds that try to fit a square peg in a round hole. If you like it, great. Do what works for you but if you play Shyvana in the traditional way, always go PTA.