Yeah, but let me throw out a scenario. Its 3 in the morning and I need some quick bitcoins. I cant go anywhere to get them except for the internet.
Buying bitcoins at 3 am using money outside of the bitcoin ecosystem is hard/impossible for almost everyone
. Personally, my only choices at 3 am are an ING Direct P2P transfer, and Paypal (both of which you must be over 18). Few Americans would be awake, so ING is almost impossible, leaving just Paypal. If I didn't have one of the top 100 reputations here, even Paypal would be hard and/or costly.
Honestly, the best thing to do is to keep more money in the bitcoin ecosystem (not necessarily in bitcoin, itself, but maybe on an exchange) just in case. At any given time, I can instantly buy an additional few hundred dollars worth of bitcoins, just off of this reserve alone.