I use direct access, as do most day or short-term traders, simply because of the commissions and execution speed. An online broker forces you to route the order through their servers, which then send it to their trade desk, which then executes it. A direct access broker gives you a direct line to the exchange; when you enter an order, you can almost instantly see it show up in the bid or ask. It's maybe only a few seconds faster, but in volatile markets it can be a huge difference.
The best online broker commissions I've seen are all about $5 per trade minimum. Interactive brokers charge $.005 (half a penny) per share, with a $1.00 minimum, while MB Trading and others charge $0.01 per share with a $1.00 minimum.
The commission and execution differences are not that important if you trade infrequently, but if you trade at least once a day, in and out, you save at least $8 - and of course a day trader making 10 trades or more a day would save $80 or more. I don't have any statistics on execution, but direct access brokers must almost certainly, because they're direct, offer better executions.
Lastly, this may not be relevant as much these days, but in the older days of the Internet when connections weren't as reliable, several veteran traders I've read about swore off of online brokers because connections would frequently go out. Of course, this can happen with a direct access broker, but I haven't heard any horror stories about them. Many (I'd say most, but I don't know for sure) serious traders use direct access. Day traders certainly do.
I don't know why people use online brokers. I can only think that it's for the research (which is of dubious quality in the first place), and because they outspend in advertising by an enormous amount.
If anyone suggests an online broker, ask why.
