well i don't think you're the only one that had that idea lol, and besides i wouldn't want to use flamethrower and immediately cancel its channel just to get the DoT. I think the fun part of using flamethrower is actually seeing the flames goes back and forth. so i think a good way to implement that would be the longer the channel, the longer the dot duration, higher dmg over time, and vise versa if shorter the channel shorter the dot duration and lower damage over time.