Superior Rune of Resistance
There are 33 signets in total in the game. Out of those, only 16 have cooldowns of 30s or less.
The one situation where you could possibly realistically benefit from a cooldown much lower than 30s would be builds that already use multiple signets. D/P Thief and Warriors are some examples of those. And I don’t think that you wanna buff either of them by giving them access to on-demand Aegis.
Right now, I use one utility signet, and it has a 24 second cooldown (normally 30 seconds, reduced by a trait). I have no immediate plans to use this rune, but it strikes me as pretty rough that I might use this signet just a few seconds before the invisible internal cooldown on the rune runs out, and therefore miss the aegis and need to wait for the signet to cool down again.