You'd stick a RIS, sight or foregrip onto your gun, or perhaps a different stock or pistol grip, so why not a flashider.
At the end of the day, all the stuff like RIS and foregrips and such are more for looks, you dont NEED them.
For the realism. Half of airsoft is about the realism. I bet if no one was allowed to use RIFs or wear camo there wouldnt be half as many airsofters as there are now.