I have 5 50's, and in spite of my Merc having a full set of healing and eliminator BM gear, I'll probably switch to my lvl 49 Marauder when I ding 50 tonight.
Here is the problem...when the game first came out, most folks (understandably) didn't know how to play their classes, so the arsenal mercs shined as the rotation is pretty easy to master. As a result, folks were CONSTANTLY crying about TM spam on the forums and in general chat most wzs. People would whine that all you did was spam TM, but the fact is, with that spec you have to get 3 TMs out before your HS missile can reach its full potential, and another 2 before your rail shot does....complaining about us using TM is like complaining about a sorc using force lightning.
Once the other classes learned how to play (in particular Marauders and stabby Operatives), mercs were FAR from OP. Classes learned to interrupt TM which really kills our rotation. One on one, most classes should be able to solo a ars merc. That being said, I still enjoyed the class and found us to be a viable part of any comp because of the solid damage we can put out (if left alone) and the knock backs make us quite useful in huttball. Unfortunately the process of making changes like this take time, and by the time they committed to the 1.2 changes, the dynamic had changed dramatically, Bioware came up with the changes that nerf a non-OP class, while leaving EXTREMELY OP classes like Marauders alone (they might have to make some spec changes, but they are no less powerful).
I still enjoy the game quite a bit and I'm holding out hope that some of the devs actually play this game, but as I said above, I will probably switch to my Marauder as my PVP main ...I looked at the Pyro spec for my merc, but it has zero appeal to me.