Well unless you're a heavy hitter with shit ton of idle cash I imagine the no commissions would be more noticed (if actively trading I should say) and as for executions they still have to hit NBBO.
ALSO... IBKR Lite users don't have to pay data fees either right?
In TWS for the Lite folks there is no level 1 or level 2 data, you have to pay for that (as of now, thinking that might change). You do get the real time quotes though.