"A laser is a device that emits light through a process of optical amplification based on the stimulated emission of electromagnetic radiation. The term "laser" originated as an acronym for "light amplification by stimulated emission of radiation"." Wikipedia the free internet Encyclopedia - original text from
Gould, R. Gordon (1959). "The LASER, Light Amplification by Stimulated Emission of Radiation". In Franken, P.A. and Sands, R.H. (Eds.). The Ann Arbor Conference on Optical Pumping, the University of Michigan, 15 June through 18 June 1959. p. 128. OCLC 02460155.
"laser". Reference.com. Retrieved May 15, 2008.
The usual energy carriers in lasers are photons of a specific electromagnetic frequency. Now photons lose energy only on one way and that is by impacting something. The problem with laser is the quality of what you use to create it, our current technology allows us only to create highly dispersed lasers which hit a rather large area with to little of photons to in astronomical terms be anything except a nice way to measure distances accurately/. To put it simply humanity has made great progress in capturing light but just a few steps in emitting focused beams of it.
But if techniques and technologies which allow much focused laser emissions would (read will be at some time in the future) lasers would be one of the beast space weapons - due to the fact that the energy loss of a laser is next to nothing and all the energy is transferred to whatever is hit while also being only slightly influenced by gravity and traveling at light speed. As we stand today lasers are manly used as latest military anti-ballistic systems, mine-clearing systems and so on. A practical space laser for combat cloud probably be build to cover a few thousands or ten thousands of kilometers but we will sooner see a pusher laser for anti-space impact defense then actual military space based lasers - we just don't have a need to put military grade lasers in space.
As for the energy dispersion - the Sun's heat doesn't disperse because most of the space outside of the outer layers of it's gas cover is empty - there is simply no particles to carry the heat. Since direct heat transfer is dependent on particles being there that means that if there is no particles there is no heat transfer. Instead of using direct heat transfer the Sun relies on electromagnetic radiation to carry heat to the distant worlds. Do note that magnetosphere of planets, atmospheres and reflectivity of their surfaces also effects the amount of energy coming from the Sun as well as the distance (due to the dispersion of particles from the Sun). But even on Pluto, almost 50 AJs from the Sun's surface around 1 W of energy falls on the surface of this world. On Earth which is 50 times closer this is over 1000 times more - around 1300 W per each meter. So the Sun doesn't give us energy by directly transferring heat but by carrying it with mostly photons.
So your points are or wrong or wrongly explained why they are valid.
Railguns are also good weapons - since they are electromagnetically accelerated projectiles there is no usual reaction to firing non-self propelled projectiles. They will also travel straight - the gravity of most worlds is just not enough to stop something traveling at Mach 6 or faster today while considering by the time we have space based railguns for long-range engagements their projectiles would probably achieve speed of few dozens of Mach more then enough escape gravity of most areas in space. In the quite realistic weapons of humanity in Halo their standard shipboard MAC cannon fire 600 t projectiles at 30 km/s (with the highest speed being 4% speed of light with 3000 t projectiles in the orbital MAC stations).
Missiles equipped with advanced guiding systems to evade getting hit with countermeasures, baits and limited AI would be also deadly weapons - they would be more like a really smart suicide robot then just a missile.
But in all space battles there would be one crucial part - your computer systems which would calculate enemy ships positions in the future (and tell where you need to fire to hit them when your attack reaches them) as well as the maximal cone in which the enemy ship cloud maneuver (until unknowingly far inertia free engines are developed maneuvering a ship out of a barrage or railgun projectiles will take hours) and so on. In the end who has the best computer system and knows his enemy's capabilities the best wins AKA in the end it all comes to how good a brain (computer) and how much information about your enemy you have.