It does have tremendous power, 25-35% CDR And the possibility to Replace Summoner Spells is Not something to be taken lightly.
But in the end of the day you have the same power as everyone else (only more often), And You are left without a keystone.
No matter how many times you Replace Summoner Spells they are what they are, and most of the time you don't Need them.
* I think if you want to use Summoner Spells as a keystone You need to give them more power and not more CDR.
For example, 30% More range, heal, shield, speed and damage, will be better than 30% CDR (With an exception To Cleanse & Teleport).
* Another option would be to let them Stacks, honestly using a flash twice in a row will feel more like a keystone.
* my least preferable option is to count time as money and when you return to the store you can buy Summoner Spells according to the time he needs, This way players will not be nervous to use Summoner Just so they don't waste the keystone.
**Reposet from boards**
I just thought it was more appropriate here in pbe