You have 2 years of those and once these 2 years are out - Blizzard will just give you other bunch of toys other expansion.
Frankly, all this "borrowed power" is not unlike talents that change between expansion or even outright various changes to core specs and classes that happen every new expansion anyway.
I actually like this scheme with powers that have expiration date, I already experienced a "snowball" design where Blizz piled on things that led to Cataclysm and absolute mess soup of spells classes with nonsense rotations and priority lists.