What would I use bitcoin for?

Incredibly easily. You simply go to an exchange and make a bid. Providing someone else has made a bid going the other way for a similar amount, you make the exchange.

That's almost exclusively what people actually use Bitcoins for, outside of Silk Road stuff. There are so few BTC stores that it's hard to justify the value of it as a currency.

It's like having a wallet filled with $100 bills at a row of soda machines.

Really? SR is the reason why?

Sales volume is estimated at two million dollars a month: http://www.forbes.com/sites/andygreenberg/2012/08/06/black-m...

Let's say it takes a week between when a person buys bitcoin and when those coins are spent.

That mean about half a million dollars of currency is tied up for these users. That's about 50K BTC. That's less than one week's production from mining.

This marketplace has almost no impact on the exchange rate directly. What it might be doing is introducing a number of people to bitcoin. They might then speculate on the exchange rate (simply by buying more than they plan to use for purchases, in anticipation of the exchange rate rising), or use bitcoins for purchases or for person-to-person payments.