I did consider using a laser, but there were several issues.
The main concern for me was safety: As far as I know, all commercially sold laser pointers have warning labels warning people not to point the product at anyone, since the light is intense enough to cause permanent damage to an individual's eyesight. Carrying a laser pointer at eye level (in a shoulder mounted weapon), and then pointing that laser at people (walking around at conventions, taking pictures, etc.), is just asking for someone to go blind.
Second, the beam width for normal laser pointers is tiny, especially compared to the width of the emitter on the spartan laser. Having a tiny beam come out of a big weapon just felt silly to me, so I decided against it.
Third, expense: A laser with an output high enough for the beam to be visible even in fairly dark environments is extremely expensive. High output lasers are also power-hogs, meaning that the batteries would have to be replaced frequently.
In the end I decided to use LEDs because they were safer and less expensive, while still providing a decent visual effect.
Now, using a bright green laser in a Covenant Carbide, that might be a good idea, as long as you don't point it at a person's face.
