If Sent/Mara are rock, then Agent is paper. If they catch a Sent out of stealth and stun them, they are going to die 90% of the time.
Sents are not invicible, just learn to play your class better.
Agents have their fun stunlocking going. I agree with most that you say but since PVP is competitive you have to compare SWTORS PVP to other PVP MMORPG titles and if you compare them, yes SWTOR's PVP need serious work and rethinking