Just because something should go down fast, doesn't mean it will. I've been playing Defender lists like crazy since Imdaar, and the thing has almost never let me down. On paper, it's easy to say "oh well you just focus him down," but that overlooks a lot of things:
1. Deployment- Rexler pops on the table at PS 8, after much of the board for most lists has been established. A smart player will deploy him where he can utilize his high speed maneuvers early to get into a good flanking position, even if it takes him a round later than the rest of the squad to fire.
2. Asteroids- Thanks to number 1, a smart player can also position him in a spot that forces the rest of the opponent's squad to be drawn through asteroid fields in order to reach him. This either controls their firing arcs, adds extra evade dice, or breaks apart the enemy squadron (thus limiting focus fire and potentially aiding the rest of your squad in selecting good targets).
3. Your Squad- If you do manage to draw the bulk of the opponents to Rexler, it's entirely possible that the rest of the squad you bring will light them up, either softening them up or killing them before they become a serious threat. And if you've planned your asteroids and maneuvers just right then...
4. Once more, with Gusto: … then your 4 k-turn leaves them completely screwed. Especially if k-turning for them would be a one-way ticket to Asteroid City. Even if they k-turn to meet you, they're now on your field as they're at a disadvantage.
However, if your opponent has planned for number 4, and come in waves/staggered advance to block your k-turn, then the 3-turn or 3-bank make excellent Get Outta Dodge maneuvers (or at least they have in my personal experience). And when all else fails, sometimes those 3 dice just love you, and you survive long past when another ship would have perished. About a third of my Defender wins have been thanks to dice luck, but it's a dice luck that is possible because of the combination of evasion and HP. You can actually survive an unlucky round to be lucky next time.