Also, I suggeste that htis may render the updgrade button near useless because we have two things doing the same thing. Isn't that somewhat redundant?
As BluePriest said, the upgrade button would still be good to have for upgrading cards you already own so you don't have to sell it and buy the upped. Also as BluePriest implied, you can't buy upgraded versions of cards that you can't buy regularly, like Nymphs.
I personally support the idea. It's only a little more convenient than going to the bazaar, buying the card, leaving, and upgrading, but little conveniences add up and make it just that little bit more fun in the end.
As in anything, once the big things are taken care of it's the little things that bring it closer to perfection. ^_^