I dont see a problem still with that change. Not like you will keep spamming hydra so now you put one, change spell and use something else. When the hydra runs out you can cast another and change to the other spell again
The problem with skills like that is the Wizard is already crazy starved for spells slots, everything in your build has to do something great or it just isn't worth it. Basically, the slot is too valuable to waste on a support damage skill.
I will definitely use hydra with some of my builds. One which includes for example the lightning hydra rune and the passive where it has a chance to paralyze. This will be in one of my pvp builds thought, but also got one melee wizard build I will try that out with