$x worth of Steem Power is actually a good idea, because it is a specific value. But i.e. 3 Steem Power would be not specific, today that would be $6-$7 worth of Steem Power, tomorrow maybe just $5, and next month maybe $20 or $25..and in a year maybe $250.
The issue is just that's not dynamic adjusted to the actual current exchange rate, that's all.