Kayle, one of my favorite champions at this time. She's very versatile and a well timed ult means everything.

However as one of the oldest champions I think her E is a bit outdated for the current state of the game and propose that you change it to a toggle. Have it drain mana on every attack like Ashe Q. Funny how one of the oldest designed champions also has an elegant solution

This would be a great change for her flow and feel as a champion, switching between Melee and Range would feel more like a constant choice rather than an obligation. It would also give her a great way to charge up a tear of the goddess.

TLDR: Kayle E --> toggle, like Ashe Q.

Would be OP... but awesome lol.

With slightly less than Max CDR, you can have her E up constantly. It's fine as is.

Not being toggle is intentional for balance and "fun-factor" reasons. There were a lot of discussions on the pros and cons of being a toggle back in Season 2 when they were doing their last major reworks. The cliff-notes version is essentially:

1. It disincentives attack speed, which has been otherwise very important. Coming to the present, this is especially true with the (sort of) nerf to Nashor's Tooth.

2. The Devs like the play and counter-play that occurs in the early game, where champions can make use of the timing of her E to punish her or (relatively) safely farm, etc. In other words, the mechanic as is adds more depth than a toggle would. Compare with Jayce or Nidalee who gain completely different skill-sets depending on their mode (and being in that mode doesn't mean their attacks cost mana), or with Ashe, who's toggle adds an effect, but a) who's kit doesn't in and of itself embrace attack speed and b) does not change such an important feature such as range.

Anyway, just some things for you to consider.