Actually the reason for laser beams being able to eat their way through metal is that the energy input is great enough for metal to vaporize from the surface where the light hits. The residual heat and the hot metal vapour then melts some metal on the edges of the hole. Photon Beam (Laser) itself has no temperature measurable. Only the frequency (color) of the light can be expressed in Kelvins, but that's not quite important. What's important is energy input into target surface - if energy input is remarkably higher than what the target can emit, the target gets hotter. If energy input (absorbation) is GREATLY bigger than what target can emit, target surface vaporizes through immediat phase change; if the difference between absorbation and emission is smaller, heating is of course slower and the target surface goes through brief liquid state before vaporizing.
Freespace beams, however, are nothing remotely like lasers. They are bound to be particle beams of some sort. Also, there is bound to be some charged particles in that beam because they are visible. Most likely some lower-end beam cannons are simply plasma held together by a strong tube-shaped magnetic field.
This is how a FS2 beam could work out: There is a cell full of main mater that forms the visible part of the beam. Then there is another cell, releasing "core matter" for the beam. Core matter must be electrically charged particles such as electrons or protons, though positrons would also work quite nicely if there were a plentitude of them...
So, the core jet consists of charged particles accelerated into great speed. The faster the better. When this thin but powerful beam of charged matter proceeds through the space, it forms a magnetic field around it, perfectly similar to the magnetic field induced by electric current in a wire. So, there we now have a roughly tube-shaped magnetic field. Through this magnetic pipe, the main damage-creating hot plasma or whatever matter is then pumped at great volumes. The magnetic field prevents the plasma from expanding rapidly into space, which it would normally do very fast. When the plasma hits the target (hopefully), it starts melting its way through it.
Pros compared to lasers:
-Looks much cooler in space, when hot matter is hot enough to be visible whereas laser beam is invisible except when it hits something
-Greater momentum effect - the mass causes damage also by kinetic energy, not just thermal energy
-charged particles ionize the target molecules, causing even greater destruction
-Limited range; the core beam starts to disperse due to electric force between particles of similar charge. Because of this the range also effects the beam's ability to penetrate armour - if the energy density drops too low, it cannot go through hull plating, causing only limited amount of damage on targets external side. When the core beam collapses altogether, the damage-inducing matter also spreads into space - this is the cause for FS2 beams ending "suddenly".
-Another thing reducing effectivity alongside range is the hot damage-matter radiating its energy into space on the journey. The cool looks have a price.
-Need of ammunition. Though the beam cannons probably are able to generate their own ammunition by converting energy into matter. Hence the advent of beam weapons after Shivan weapons' research only; earthlings and vasudans were not able to create (or even think of?) such possibility - the Shivans brought the beams to us.
Strange thing is that Shivan beams are red, which is the lowest energy color in electromagnetic spectrum. So I bet the Shivan beams get their power mainly from sheer volume, whereas Terran/Vasudan green/blue/silver beams get hotter/more energetic and thus don't need that much matter into them.
Of course, this is just speculation. Main point is however that beams are _not_ laser beams. They shouldn't even be mentioned in same sentence. Good example of how ineffective laser weapons are is ML-16, and even that is quite too effective considering the energy drain mentioned...