Here's the thing about the Anzu's admittedly somewhat weak arms: It can still dish out meaningful damage. If you shoot off the UAC-10 arm, you still have a LRM-10, Light PPC with Capacitor, and ER Medium Laser. That's 6 average + 5 or 10 + 5 damage... still enough to force a PSR if all of them hit and the LPPC is Charged at the time. Even without that it's still 16 damage total. That's not great, but for a unit that has lost its main gun that's not bad.
If you lose the other arm, you still have a UAC/10 and LRM-10. 16 to 26 damage. That's fine.
Yes, I would have rather seen the design lose TAG in favor of bringing the Arm protection up to 20 and finding something else to do with the remaining half ton. What we have is still playable and pretty strong, though. The Anzu's response to losing an arm is to simply set its other weapons on their higher damage, higher heat modes and still do decently well.