If speculators are correct that it may go up in the future, it is in their best interest to bid the price up now and collect money in the future. That would make BTC not a speculative bubble, but merely an asset that speculators have properly valued.
If you want to argue BTC is or is not a bubble, do it right. Choose a theory of money, e.g. Baumol-Tobin. Then come up with predictions about future transaction costs, the size of the BTC NGDP, and the price level. Finally, use your theory of money to predict the value of BTC.
If that disagrees with the current speculative value, you've got a real argument.
(For those unfamiliar with the macroeconomics terms I used, I explain it all here: http://www.chrisstucchio.com/blog/2014/demand_for_bitcoins.h... )