Some folks are only comfortable using the biggest names in the business ... the ones who advertise on tv and in mags etc. etc.
They are willing to pay several times as much per trade.
Many people don't have the time to use tools like Telechart 2007 and Blocks, or the inclination.
Many don't even know there are advise services like Schaeffer's Research and Gorilla out there that have different programs to subscribe to in order to get trading ideas / suggestions.
There are lots of reasons why people use expensive trading services.
Interactive Brokers isn't for amateurs, but for anyone with a bit of experience it is a great way to improve your profits.
I've been with IB for several years and love it.
All serious investors should check it out