I was in an explore mode dungeon with another thief who questioned why I'd ever go melee given thieves die in one shot. It made me want to quit right then, thieves have great survivability if they're not built as glass cannons (which doesn't make any scene in pvp). On top of that they've got enough blinks/escapes that getting out of situations when they get hairy is never too hard.
Well there is a popular glass cannon builds with heartseeker for pvp, probably the only build they know.