Hi!
I am with the magma-is-too-much-trouble faction.
First of all, I don't have a murderous fortress, so I don't set up things to kill civilians and traders. And for everything else, there are the good old weapon traps, thank you. (And if I were to challenge something along the lines of forgotten beasts, I would probably go for a cave-in trap) Thus, magma is not useful for me as a weapon.
Secondly, I am more interested in playing Dwarf Fortress as a game of construction rather than survival. Thus, I prefer easy sites, usually heavily forested with lots of plants in a temperate to cold climate. Thus, I have a basically unlimited supply of wood that can easily fuel my furnaces.
Third, I tend to play with a small setup, trying to get few immigrants during the first year, and having a low population cap (< 7). Thus, I have very slow fortress growth which gives me time to hoard fuel. In addition, this also increases the effect the regular import of bituminous coal has on my economy.
For these reasons, there is no real need for magma in my fortresses.
On the negative side, there are the following things:
Dwarves are not careful enough when handling flames or when ablaze themselves. This makes magma itself a highly dangerous substance that can easily ruin an interesting fortress.
Secondly, magma is known to be inhabited by creatures that also wield fire. So, even when carefully designating your mining, there is a risk involved with breaching the magma.
Third, and this only applies to the most recent version, of course, breaching the magma requires first breaching all the cavern layers. And personally, I consider the cavern layers a very nice, game-like feature of the game where the player can freely move on to the next challenge when they have established their current situation. If I made magma important for me, I would basically forfeit this freedom of setting my own pace for the challenges of the game.
So, for me that is 0 pro and 3 contra arguments, leading to a clear "Nope.".
Deathworks