CLS lead is not for the 50% chance to counter. It's mostly there for the fact that rebels gain 5% turn meter for every effect that is resisted by the enemy team. And with OB, that is tremendous turn meter gain, since he inflicts potentially 10 debuffs. Even if a few of them are resisted, OB and CLS go almost immediately after that.
Remember that OB himself also gives 3% TM for rebels on enemies resisting effects, so it's actually 8% turn meter gain for CLS and OB for every effect resisted. It makes them both go extremely fast.
Dodge / evasion doesnt really work well in the R2 world. Plus there are toons like CLS and Wampa that give accuracy up. And defense up is only useful if Barriss is also in here. But having two tanks spreads the damage out, and surviving is not really an issue. What is more important is that your guys go fast and go often.