Go try pyro on a powertech and mercenary both, then tell me how you feel about that being the mercs best PVP option.
I'm dead serious because I agree that Pyro is probably the better tree for merc on PvP DPS. If you try it on both ACs, you will see the problem pretty easily. Whether you make it work just fine as merc or not.
Don't agree after 1.4. Powertech range nerf is a serious disadvantage.
I constantly monitor range, and I try to stay as close to 30m from everyone as possible. Even versus powertechs, I can now more than hold my own by maintaining range above 10m.