For 4 points and a training slot this upgrade allows you to add a red die to your melee attack while your unit is damaged or has had a model destroyed.
At 4 points Tenacity is cheap for a training upgrade slot, and adding a red die in melee is quite valuable - but the additional cost of having a wounded warrior can be steep.
This upgrade is recommended on every melee centric unit that can take it, especially if they are multi-wound models. The function is quite straight forward, but the added attrition value and damage output of the unit greatly outweighs the 4 point cost on the right unit.
Wookie Warriors Expansion, Royal Guard Expansion,