It is a vaguely cool interaction, but the implementation is weird. The fire DoT is much shorter than the trap duration, doesn’t refresh if the target stays in the trap, only applies to targets standing in the trap when the Flare is tossed, and (in Torghast at least) does pathetic damage (160% AP, a bit less than 3 unbuffed non-crit Arcane Shots). The legendary version does 900% AP, about 3.333 Aimed Shots, so enough damage that you want to use it all the time for something like M+, which is not great IMO.
Agreed. Having that would make MM feel awkward having to pause every 30 seconds to throw out a tar trap and then a flare.
Now if the legendary also made that interaction automatic, probably wouldn’t be too bad. And by automatic I mean, the tar trap sets on fire as soon as it activates.