There are three broad groups of people who actually need bitcoins:
- The young, who don't have credit cards and maybe not even non parent controlled bank accounts but want to send someone money. What better way than bitcoin?
- The embarrassed, who probably don't want payments to a porn web site appearing on their credit card statement.
- The 'legally challenged', who want to order illegal goods, pay for an illegal service or just want to move money with an added layer of obscurity (tax evasion, money laundering, etc).
The rest don't really matter. Bitcoin will never be popular as long as it's a geek's toy, or used on the basis of libetarian ideals. I expect the above three groups to drive bitcoin's adoption (or failure, if they're just not interested).
Those might be the drivers at this point, as libertarians ideals and geek interest were the drivers when bitcoin was just an idea. But when/if the Bitcoin economy develops due to those drivers, Bitcoin will become a very real and efficient way of sending money around the world and as it grows more merchants will see the utility of accepting it.