There's a good reason for spending this money in the beta, and it's something I'd encourage you all to do as well.
First and most importantly, you're going to get new packs of cards when the game goes live. All your packs of cards from the beta are going to get deleted, but you'll get an equal number of packs to open up when the game goes live. Whatever you put into the beta money-wise will get transferred to the live side. That means when the game launches (whenever that may be), I'll have $150 worth of cards to use out of the gate.
Secondly, having this many cards allows me to test the game in stupid ways. I can do crazy things like disenchant legendary cards and purples -- which you'd never see me do on live. I can figure out the best cards to make without having to really consider the monetary implications. It's all there, essentially a sandbox, for me to do whatever I want. Now of course this is part of my job, so there's that factor -- but I'm also the snobby gamer who likes to min/max everything possible. The beta is a beautiful thing for me.
Third, and perhaps most importantly, it lets me try a wide variety of cards and decks that I won't be doing in live. When live hits I'll be specializing my play into two or three primary decks and going with that. It's unlikely that I'll be doing more -- maybe at some point I'll turn into a collector, but I doubt it. In the beta with my purchase I have the freedom to figure out what's what in a safe and no-pressure environment.
Now you might be asking yourself why I'm writing this... it has to do with the reaction to paying money into a beta game. A lot of people in The Queue (our daily Q&A column) ask each day about buying cards in Hearthstone, and the questions inevitably turn into a debate on the merits of spending real money in a beta test. There's some clarity needed here, and I hope this helps with that -- or at least people take away two things: no one is forcing you to spend money, and the money you put into the beta test will carry over to the live game on release.