No dumb if you get to play for free and want to know the inside track intimately before the launch.
Also, if you are a bottom feeder who makes money off of selling in game resources it will be very valuable to know the best ways to level up and secure the most cash per time ratio and where the good tradable loot is.
Someone with that kind of money for the beta account either has so much cash that $600 is nothing to enjoy a title that has more hype than availability OR they know they can recover the cost by using this time as research to corner a market.
If a kid who works at 7/11 saved his pennies for weeks to buy the account then yea, it wasn't the smartest thing to do.