I think it would be awesome to be added into vanilla. I play with Masterwork which has them, and it makes perfect sense!
First, they are hard to make. You actually have to progress through some research before building the workshop and then building them. It's a later game thing and makes ranged combat a bit more interesting.
Secondly, dwarves are all about explosions, mechanical things and contraptions, so why wouldn't dwarves like a metal contraption that exploded through the use of mechanical things?
And lastly, these are MUSKETS we are talking about. Urist McSwat Cancels Kill Azgh Sgbuletinspine : Already fired the 100 bullets in his MG. The dwarves shouldn't progress beyond muskets, flintlock pistols and cannons, unless Toady starts adding in the Space Age. Muskets are rather inaccurate, and in open combat, someone might be able to get off one shot before they are hacked to pieces by the goblins. So it would be best to place a line of musketmen on the walls, and watch as the explosion cascade! Ha!
Actually, talking about explosions, it would be rather cool to see muskets generate smoke, so when used underground, smoke starts to fill the corridors. Could lead to some interesting fights where the dwarves tried to hold off, but were blinded by their own smoke and eaten by Antmet.