First off, even with a massive flamethrower, which would incredibly limit your mobility, you'd have to come super close to hit your enemy. Second, fire would do constant damage if you want it realistic. Imagine setting the enemy on fire and his health decreases for the 30 seconds. The riot would be on, I tell you.
But seriously, some need to think first. Lately someone asked for molotov cocktails. Like, surely would use a molotov cocktails when I've got access to grenades or even a grenade launcher. Long story short, do not want.