I asked the same thing @Kookermader and @BrakirKY posted the below well thought out post:
If you're running a resistance or hybrid resistance/rebel team with him, then his unique is most definitely worth it. Even if you don't run R2, it will still benefit C3P0 by getting him to the 3 stack CD reduction bonus.
Example:
R2 starts with 1 stack of translation ( SoT )
1st turn he uses smoke screen and gains +1 SoT, giving him +15% CC
2nd turn he uses his burning AoE, which is special damage based. Since r2 has a fairly low base special CC under JTR of 55% ( around 25% at g12, with 30% from JTR lead if using it), that extra 15% could mean a couple extra exposes to generate more TM for your squad.
At the 3rd stack, CD reduction will happen whenever c3p0 uses his basic, allowing more stealth and burn more often. It doesn't seem like much, but it will get your team rolling faster, which is always a plus.
For TB, the stacks of translation reset every wave, so it will help there also if you use him.