Maybe because the plasma weapon bodies themselves aren't made out of metal, or of materials which have a higher melting point? Spikers aren't modeled after Forerunner tech after all.

Quote Originally Posted by Inferno View Post
That is a really cool way to use the A-Out functions. If the warthog didn't already have 4 functions that would look cool on the barrels of the chaingun.
Nonsense, the warthog gun weapon only has A and B-out in use. C and D are free.