I wonder how you see the sniper rifle with this viewpoint. I don't think the splaser is perfect, but like the sniper or any power weapon, it is a tactical advantage that should be fought over and captured before your team decides to take the risk going out into the open or otherwise face the consequences.
The key difference for me is that the sniper is hitting much smaller targets for an instant kill, doesn't have a giant beam that (at the very least in Halo 3) was practically a shotgun of smaller beams, and isn't capable of getting multi-kills for each shot.
I mean, we could take the "anybody can control it and it should be factored in" argument to the extreme with an example of a souped up UT Redeemer that obliterates a quarter of the map and it would theoretically hold. Something like that might be balanced in a technical and rhetorical sense, but that doesn't preclude it from not being any fun to go up against. When I'm killed by the Laser without any warning, it's annoying. When I'm killed with the Plasma Launcher because I failed to react properly, I realize there was something I could do to avoid it, just as there was something the attacker could do to counter my evasion.
I've got about 200 kills with the PL in just under 300 Big Team games. Considering it's on what, three maps in that playlist, I don't think it needs a boost. It's situational, and has to be treated as such. The laser, on the other hand, generalizes extremely well to infantry. Expanding on what I wrote earlier, the Plasma Launcher lends itself very well to ambush situations, but in a good way: the ambusher has to be within a reasonable distance for it to work. The laser is a crossmap ambush: you could be rounding your own base and get blasted to hell instantly from theirs.