i guess. i fell that if i dont have the devistators im going to have no long range. l have 2 missile launchers and two heavy bolters. so il only be able to shoot with any power at 24'. so il get 2rounds of fire before im swimming in gaunts. and if the vidicators miss then zoanthropes will shread them.
Well, it's true that Devastators has more range, and more missiles, but it has much lower stopping power.
Let's compare the weapons :
Frag Missile : 48" S 4 AP 6
Demolisher : 24" S 10 AP 2
Demolisher cannon has a bigger template and much more destructive power. By hitting with a much larger Strength, you have a bigger chance to wound the enemy. Even you can kill the Zoanthrope if you hit them, and it fail it's invulnerable save. Demolisher cannon is also have bigger template, so more targets will fall under your fire.
Don't forget that Vindicators can fire and move, while Devastators can't do that.
And as far as I know, you can only target one squad, by having 2 ML you can't target 2 enemy squads, am I right or not?