I disagree that DF is not viable. GS role in PvP is applying pressure on distant targets to whom it's hard to get into melee range (usually healers), or just applying overall pressure to the enemy team from afar. SS is good for that purpose, but DF can do one thing better - moving around. No healer will just stay casting out in the open (esp. Scoundrel). As DF it's much easier to follow the healer all the time while still dealing damage, and apply burst when you catch your target rooted/CCed.
As for breaking mezzes - well, you gotta know what you're doing. Over 50% of DPS classes specs have a DoT in their rotation, so discarding them because of that and saying that they'll berak mezzes - that's not correct.
In some cases I would prefer SS for the upfront coordinated burst, but in the balanced teams there are other classes to fulfill that role (Assault VGs, Smashers).
Pressure on healers? MM is a pressure tool because it threatens any moment to overwhelm with its burst. Healers are not afraid of DF. 2 out of 3 classes can cleanse the dots into their weaker form. Ask any healer what he is afais of more and he will tell you: marksmanship.
As for following healer behind a corner, no way... This is never a snipers job. A sniper out of over is a free kill for anything with a leap or pull. If you are going into enemy territory to ge that elusive heler armed with only 2 dots, you subject yourself to high risks.
Let the melee, pt and concelment ops follow healers behind corners. Your job as a sniper is to make sure that once he appears in your sight he risks getting killed. Its a turret class, and thus its best at area denial.
Dont get me wrong, the spec is good at what it is supposed to be, a tank killer, a damage machine, but its such a specialized role that you will often feel lacking something, either burst, or control, or help from team mates.