edit: To a certain extent I already can. I've been paying for web hosting with BTC for a couple years now, I can buy electronics and certain other miscellaneous items on Overstock, and there are plenty of other places online (sadly, no local brick-and-mortar) where I can spend them. I'd think with global adoption, though, the value (in terms of buying power, or USD/Euro price, or however else you want to measure it) would be a lot more stable.
The problem I fear is that in reality these retailers are just using Coinbase/Bitpay to convert it back to fiat - meaning to them their just considering it as a foreign currency transaction. This isn't really that useful - wha you'd want is for them to keep the Bitcoin and use it to pay for wages and shipping etc. making it an actual currency rather than just a commodity. Unfortunately it is a catch-22, someone has to be willing to go first and have the gall to keep Bitcoins rather than converting them back, but no one wants to do that since nobody else has done it.
They don't do this because too large exchange rate risk, even during the one hour time when the trade is under confirmation, the exchange rate will fluctuate wildly thus cause a loss (or gain) for one of the trade party. In fact, even if they immediately convert, some of the coins are still needed in the transaction, that will raise the demand, also helping expanding bitcoin economy
We are seeing a huge currency war around the world, and during this war, some of the small fiat currencies swing wildly similar to bitcoin. The merchant of those country might prefer to hold bitcoin to avoid the volatility in local currency, especially international importers/exporters
