The commissions are important part of every system and strategy :) Lets say you are using an Expert Advisor on a commission based broker, providing interbank core spreads. The Take Profit of the EA will be activated early because of tight spread / or the Stop Loss will be activated bit later because of tight spread. Generally this could be helpful for the overall performance and more profitable. On the second scenario: you are trading on marked-up spreads, without commission. In such case, it could take more time to be activated the TP (because of wider spread) / or the SL can be activated early (because of wider spread). This could generate you bigger losses because of earlier activation of the SL and because of bigger chance to skip activation the TP.