Before something can be a store of value, it needs to have value. Gold gained this initially through being wanted for use in jewelry. The only way bitcoin can gain value, is by proving its worth as a useful (or even best) means of electronic payment.
This is a good point. Can bitcoin be a reliable store of value (as it currently is) without being supported by its use as a highly convenient electronic payment method? Can something be established solely as a store of value? Where does the value come from? I'm guessing it being a highly convenient store of value... But I can't wrap my head around it to be convinced that it can be enough...
The answer to this actually answers the OP question of why it's important that Bitcoin gains prevalence as an accepted method of payment.
What gives currency value is the existence and high-availability (currency liquidity) of transaction counterparties -- that somebody always wants to be at the other end of your transactions. The second you can't find a buyer for your Bitcoins or a seller that will let you buy into the Bitcoin system, Bitcoin no longer has value. After all, the value of a Bitcoin (like the USD->BTC conversion rate) is a direct measure of how many people want Bitcoin.
Until there's a commercial market for Bitcoins, all Bitcoin counterparties are fellow speculators and investors and the Bitcoin exchange is fueled by fickle things like novelty rather than reliable things like food and the human desire to buy things that we don't really need. The intrinsic value of a currency isn't its ductility or conductivity (i.e. gold) -- they are flatly irrelevant --, but actually the reliability and ease of finding a counterparty to your transaction. A commercial market is what entrenches the transactional reliability of a currency. It creates cycles of exchange and networks of trade that entrench and distribute the investment of the currency across a lot of people, which increases the chance that there's always someone out there that wants to trade with you.