I use them because:
a) for the most part, the numbers on the card (attack roll, weapon die, what is added to damage, stat mods) are things I can keep track of on my character sheet, and grabbing the card is easier than looking it up in the book.
b) I'm lazy

I'd rather not print the cards out invidually. Also, the card stock is a bit nicer than normal paper, so it's easier to use them over a long time, and they can be reused/retrained without reprinting.
Basically, not having the exact numbers on the card isn't too bad, as long as you use the attack/damage calculation boxes on the front of the character sheet, you don't have to do a lot of extra math for what your powers do.
However the fact that it's only PHB powers at the moment does make for them being a bit less useful depending on the situation (I am running for one group that it's entirely useless for, and the other group it's only really helpful for the wizard ... but in the wizard's case it's
very useful since each power attacks different defenses, has different sizes, is close/area/ranged, etc ... and the player often has a hard time remembering the powers, so it has a combined benefit of letting them see all the powers they have available without looking through the book (not to mention it helps track what is or isn't used).
Once the power cards are out for martial power, the martial classes will probably be more or less fine since there would be few powers outside those two books they will care about. Luckily most classes would only need to worry about 2 boxes (PHBx and Y Power) to cover most of their powers. I wonder what they will do with say ... the swordmage's powers from Fogotten Realms.