Because considering the stealth detect range of perception and paranoia stacked together means you WILL see that rogue coming. It's totally viable, you just need to be very well aware of where you place yourself, and with whom.
In pvp Warlocks are a support class, I'm sure nobody will argue that. Solo Warlocks are dead warlocks, geared Soul Link Warlocks aside, and even then a buddy with heals just blow the enemy out of the water.
In the end it's all situational, more often than not I see the rogue headed towards me before he stealths, and having played one I suppose it's an advantage to guesstimate their actions..
At any rate I would rather have a chance to be offensive against my counter class and kick them in the nuts with dots while they're doping around in stealth chasing after me, than plan on using my racial (1 min, & yes its a very nice one I totally agree) and then just die after the stunlock ends rendering it useless xP
Edit for Diplomacy its a damn good racial, you gear faster, means that you pewpew that much sooner yay