I can't see any downsides to putting this in.
- More Dev time to figure out the best key, code it, design art for the UI to accommodate it, and test the new functionality. That's an opportunity cost paid because you now can't use that time to design other things.
- Potentially messing up some people's keybindings.
- Creating a new button that isn't used for anything else in the game, and will be used by at MOST one person per game.
The best argument I could see is making Rift Herald an interactive icon like Kindred or Kayne have, where it's a special button on your UI that you click, rather than a specific key. That has the minimum impact on the game, although the tradeoff then is that you lose the interesting choice of "hold Herald or regain vision tools?" I'm not sure how valuable that decision IS overall (would have to analyze game trends), but it's possible that if accidentally Heralding is a big problem (and I'm not convinced that it is) that might be a good solution.