I've seen folks on Q2Chaos servers just come on and run around and throw poison and flash grenades all night and nothing else. Of course in Q2 there was the unfortunate fact that folks could go into their ini settings for the game and turn off these effects. So they could spam flash nades and poison and never have the effect on themselves. Sad.
This is also related to why you only get six of them at a time max. To prevent spammage. And why they have some colored trails, to keep it fair, offer a chance to react.
Anyway, you sorta answered your own situation. You do know it's a flash grenade, so you can turn your back, or you can toss it thru a door and step away until it's done, or toss it thru the window right before your teammate goes in for the enemy flag, stand on top of your base and lob them down in front of attackers, or whatever.Wish the shooter were immune to the effect as he knows he's shooting a flash-bang, the target does not.
But as far as close quarters DM fighting goes, no, it is not perhaps as useful there. But I'll still do it sometimes just to break up the pace on somebody who is on a roll, or even just for fun once in awhile.