When it comes to the main reasons to fire up your special, I feel that the Sting Ray doesn't do any of them as well as any of the others (bar maybe the Inkjet, but I just suck at using the Inkjet).
For one thing, it doesn't spread ink at all. Sure, most of the others don't spread a lot of ink, but it's enough that it can give you a bit of an advantage in a duel. But okay, that's part of that special, fair enough.
But the other two main reasons to use specials (raw kill power and pressure) are handled way better by other specials as well. It kills way too slowly to be any real threat to competent players, and you have to stand away from the battle, or else people are just gonna come up to you and splat you. So, you'd have to use it form a distance, but it's not reliable enough from a distance. If you use the Tentamissles (which doesn't excel in raw killing power or turf spreading either), you're done in 2 seconds and can actually use your main and sub to interact with those missles. Not to mention the missles themselves require more effort to dodge, since most of the time you jump over the Sting Ray, that's the last you'll see of it. Plus, it's way easier for a teammate to make use of your missles than of your ray.
Splashdown is a great panic button and manages to kill pretty much everytime I use it. Ink Storm is very easy to use as a pressure tool. Bomb Rush creates chaos all around and helps spread some turf. The Sting Ray, on the other hand, specializes in nothing whatsoever. Yes, it's got range, but like I said, it's too easy to dodge and takes ages ro rank up damage. The rare times I get killed by a Sting Ray I'm practically embarrased. I kill a lot more people WHO ARE USING the Sting Ray than Sting Ray users kill me, and I'm sure that goes for a lot of people here. I mean at like a 5:1 ratio, at least. The same goes for the Ink Jet and the Curling Launcher, though in latter case, that's probably due to incompetent players more than anything else.
Anyway, if you like the Sting Ray and get use out of it, great. I can't though, and most players I play against can't either. Maybe it has its uses in Tower Control and Rainmaker (especially the former, I can imagine), but I don't play those modes.