The Blaster Launcher is a phenomenal weapon, it instantly transforms the most dangerous enemies in its range into relatively safe encounters. Let's see what a heavy with Danger Zone, HEAT ammo and Rocketeer can do, and remember, the Blaster Launcher always hits:
Groups of elite mutons - take the lot of them down from 14 health, enough to survive anything except a plasma weapon critical or a plasma/alloy rapid fire, to 5 health, easily killed with any plasma weapon or Mindfray. If your other operatives have a good chance to hit, you can kill them all before they even get a turn.
Sectopods - deal 18 damage and kill both support drones, leaving it with only 12 health. This means you only need 2-4 operatives to use their turns to kill it, freeing the rest to deal with other threats.
Cyberdisks - instant death.
Groups of chryssalids - instant death.
Ethereals - 9 points of damage out of a total 20 with no chance to reflect. You haven't felt pain if you haven't had a plasma bullet swarm reflected at your heavy. A solid opener or a lifesaver if it's your last move. Very likely to inflict grievous harm on the Ethereal's buddies.
The final confrontation - guaranteed win in the first round with 3 blasts, and the 3 spare rockets from Rocketeer mean you can dominate the opposition leading up to it.
I believe taking Mayhem instead of Rocketeer will let you kill most enemies outright, including Sectopods if you have HEAT ammo, but the extra rocket is too good to pass up, in my opinion.
Now, for the most important feature of the Blaster Launcher - the projectile will route around obstacles and even break windows to reach its target, this means you can place the devastating explosion exactly where you want it, instead of only where your heavy has line of sight. This feature instantly transforms your heavy's secondary from something you use every now and again because it's almost impossible to get a good shot into a major staple of your tactics.
The transition from Rocket Launcher to Blaster Launcher is the single most important weapon upgrade an operative can get, comparable to going from regular weapons to plasma weapons. Can you complete the game without it? Sure, though the Blaster Launcher can guarantee a win, while most other weapons only give you a fighting chance. Is it worth delaying the final mission to get it? Doing so will expose your operatives to quite a few dangers, so it may be prudent not to risk them if you do not have replacements, but clearing a Battleship is a nice achievement in itself and the prize is worth the risk if you have replacements.
