For consistency, if costs written with numbers are replaced with symbol. Also eliminates some confusion (that is, if I wasn't the only one who was confused when I first used cost cards...)
Bah. Consistency. This isn't a pudding, after all. =P
Seriously though, there isn't any reason to make it a functioning symbol. Any confusion about it should be resolved within the first couple of days of playing, if a person is paying any attention at all. Besides, unlike other PQ symbols, this one is (or rather, isn't) already in the game in a functioning manner. Why mess with something that already works, and have to go back and rework all those cards currently live for very little, if any, gain?