Guild Wars PvP standard is: Jump in, spike the squishy, get out.
Regardless of tanks or aggro-control, the moment the Sin appears, he is gonna aggro SOMEONE on account of generally going straight for the healer (shadowstep techniques help)… so the trick is to do the damage (i.e. assassinate the healer) and escape before they can react. No Sin is likely to last long in a drawn out fight behind enemy lines, irrespective of what the allied healer or aggro-control is doing.

Mabinogi was… different…
No strict classes as such, though most people specialised one way or another. As such, you’d only have a dedicated healer in the party if you were lucky enough to know someone who voluntarily specialised in it (they tend to be VERY appreciated though). Folk in Mabinogi are generally expected to keep themselves alive, unless they’re up against insurmountable odds (in which case there is generally a healer just outside the room, healing them up from outside aggro-range).

Also, I always preferred having some back-up healing option to relying exclusively on healers. If losing the healer equals instant party-wipe… it probably wasn’t a very good party in the first place.