Living Bomb is not a lackluster raid spell as is claimed in the latest edit.
A Mage's DPS is the total damage done divided by the fight's length. From a pure fight DPS standpoint, a spell that does X damage over 12 seconds is indistinguishable from a spell that does X damage instantly with a 12 second cooldown. They both do X damage the same amount of times. I will treat LB an instant with a 12 second CD for simplicity.
Take X to be LB's total damage over time. X takes 1.5 seconds (the GCD). Fireball takes 3, meaning X has half its cast time. If X's damage it is superior to half a Fireball's damage, casting it is a DPS increase. And it does, so it is. Simple as that. The real question is: Is it a sufficient DPS increase to warrant the talent point?
Take Living Bomb to be a spell with a 80% + 40% coefficient (taken from spell power coefficient) and 1380 + 690 base damage. Fireball has a 115% talented coefficient, 1010 average base damage and a 12% damage increase from Torment the Weak, and twice the cast time and better crit scaling. Both spells have a crit modifier of 245% (Ignite + Burnout).
Now take a Mage with 2000 spell power and 40% spell crit when properly buffed.
Living Bomb's ticks do a total 1380 + 1600 = 2980 damage, while its explosions do (690 + 800 = 1490) + (1490*0,4*1,45) = 1490 + 864 = 2354.
5334 damage over 1.5 seconds, or 3556 DPS.
Fireball does 1010 + 2300 = 3310 x 1,12 = 3707 + (3707*0,4*1,45) = 3707 + 2150 = 5857 damage over 3 seconds, or 1952(,333) DPS.
Casting LB results in a DPS increase.
Analyze now a situation in which 8 Fireballs are cast, 24 seconds, versus one in which 7 Fireballs and 2 Living Bombs are cast, also 24 seconds. 1952 DPS over 24 seconds in one case, 1952 DPS over 21 and 3556 over 3 in the other.
46.848 damage in the first case, 40.992 + 10.668 = 51.660 in the other. The second situation = 110.27% of the first one's damage.
Living Bomb has increased DPS by 10%.
This is all in pure example, and crit rates can fluctuate, going higher (lower LB contribution) or lower (higher LB contribution) than 40%.
LB is a decent single-target damage spell and that is its main strength. Its damage contribution is not lackluster and raiding without it shall not be recommended by a responsible wiki.