Sure, but why can't I transfer money to you, receive confirmation that the transfer has completed, wait some reasonable amount of time (no more than a month maybe?) and then convert to BTC at the exact moment I prefer? Then I would feel like an investor, rather than a chump.
EDIT: actually it would be nice if you responded to some of the many seemingly-reasonable customer complaints on this page.
They on purpose don't allow balances in USD as maybe then they will be considered a bank. Once you go thru their verification process, you get buy instantly up to 10 BTC/week without waiting (although sometimes they turn this feature off for whatever reason). Also, you don't need to wait a whole week - you get hourly increases of the 10 BTC quota, which is nice... when it works.
EDIT: actually it would be nice if you responded to some of the many seemingly-reasonable customer complaints on this page.