Shield Probe says "hello". Off the GCD already, more healing than Unnatural Preservation (for a DPS sorc), and half the cooldown (for Lethality).
Sure, but now you're comparing ability versus ability when you should be comparing the whole toolsets of the classes (or at least specs) against each other. Just add in Force Speed versus roll and things are already skewed in favour of sorcs. In my experience (which admittedly is limited to yolo on both scoundrel/operative and sage, as I have only played team on other classes) both classes have ways to deal with focus, but my sage loses far, far less output from it. In fact, it is probably best for your team if a properly played balance sage is the one to be focused, because it loses very little from being focused compared to every ranged and most melee specs in the game (before the patch anyway; it might be different now that more emphasis is added to TKT - I haven't been able to play since it went live). Telekinetics is another story, but the reason it loses output from being focused is in very little part because of gcd demanding defenses.
Actually, Lethality has fantastic sustained defense, it just requires a lot of micro-management to get the most out of it. Relative to the sustained DPS levels in PvP (i.e. very low), Shield Probe is straight-up OP. You're absolutely right though that burst defense is where Lethality is lacking. Evasion helps, but only against certain classes (basically just Ataru sentinels and DoT sages). Hopefully they still plan to rectify this situation.
That "fantastic" sustain is dependent on just how much output the other team puts out, and how the damage is spread. Against proper hard swapping sustained survivability means close to nothing compared to burst defense. Against any properly executed strategy that involves a semblance of focus - no matter how simple - sustained survivability is just unused potential. Heal based defenses on short cooldown relies too much on them always being used on cooldown to full effect. If those terms are not met, the "fantastic" part of "fantastic sustained" falls apart. As far as pvp goes, sustain only shines against a pressure comp or bad opposition.