It depends on which tank and it's role.
I like to place them on rhino in which I place units which stay behind to claim objectives. For example.
5 NoiseMarines, 1BM, 1SB Rhino + Havoclauncher. This gives you for just 195 Points 1 blast Str8, 1twinlinked blast STr5, and 3 S4 Shots in a tank which can claim objectives.
I put a havoc launcher sometimes on a dakka pred:
Autocannon, 2HB, 1 Havoclauncher, gives you for just 115 pts a good way to kill incoming transports and hordes.
When I field PMs have have usually 3 units of them. Two with 2 melter each and one with two plasmaguns. The guys with the plamsaguns stay back and claim an objective far away from my opponent. In order to let them still shoot something their rhino is equipped with the havoclauncher.
I like to place them on rhino in which I place units which stay behind to claim objectives. For example.
5 NoiseMarines, 1BM, 1SB Rhino + Havoclauncher. This gives you for just 195 Points 1 blast Str8, 1twinlinked blast STr5, and 3 S4 Shots in a tank which can claim objectives.
I put a havoc launcher sometimes on a dakka pred:
Autocannon, 2HB, 1 Havoclauncher, gives you for just 115 pts a good way to kill incoming transports and hordes.
When I field PMs have have usually 3 units of them. Two with 2 melter each and one with two plasmaguns. The guys with the plamsaguns stay back and claim an objective far away from my opponent. In order to let them still shoot something their rhino is equipped with the havoclauncher.