The more brokers using the MT4 api the better. The more people using it the more user support.
It's nice having a FREE fully functional interface, even if there is an occational bug, they are always trying to fix it and putting out new builds.
If they wanted to steal peoples traders, they could easily program it into the code to email them any mq4 file that is making a ton of money.

just my 2 pips...

-Adam