Competent player > Incompetent player.
Look up Confirmation bias, I think you suffer from it.
Mercs are support role class, people aren't playing them correctly.
Says who? You?
They're relegated to a support role due to their terrible current design. I've never heard a SWTOR Dev state Mercs are suppose to be a support class. If they're a support class, where's our support utility? It's non-existant. You can claim we're a support class all you want, but if we're going to be a support class, then where's our support capabilities? How do we support better than any other class? We don't.
In good hands like yours, they can be absolutely devastating and Pyro Mercs are very viable. I have a guildie who runs it and he tops the DPS charts as well.
You have no idea who this guy is, he makes one post in favor of Mercs and he's your new best buddy.
No one said Mercs can't top the DPS charts (which are somewhat irrelevant as this is an objective-based PvP game). It's easy to top DPS charts when you're being ignored. The guy even said he faced pugs, who were likely terrible.
Arsenal has it's own issues, but I've seen Arsenal players do extremely well.
Against bad players? Sure. Arsenal spec is the easiest spec to shut down in this entire game; by saying you've seen them do well, you're pretty much admitting they're playing against bad players. If you're admitting that they do well against you, then that says more about you than them.
The problem is that they think that they should act like a rotating turret. It needs to be a TM TM TM, move, railshot on the go, Proc'd Unload. The more mobile you can make yourself, the more effective you become. Hug corners. Little things like that.
Regardless of how much movement you want to toss into the mix, Arsenal is an incredibly immobile class. The fact is, they're dependent upon TM and Unload casts, which are easily interrupted. You can throw a sticky/railshot on the go, but outside of that, you're not doing much.
The class is messed up as a whole; pyro spec is slightly more viable, but it's not nearly as good on a Merc as it is a Pyrotech. I think you have a problem admitting that Mercs are in a bad spot. Do you still feel accomplished when you kill them? If you do, you shouldn't.