Missing power pin may affect the chip's stability. It would depend on the design.
Basically, each power pin is designed to handle X amount of current flowing through. Now the chip will draw Y amount of power. Based on how much power it will draw, there needs to be enough power pins to handle the current. By power pins, I mean VDD and VSS. They usually come in pairs. I don't know how pairs the Xeon has. If one of the VSS pins are missing, then the current will have no where to go. It kinda defeats the purose. It's like having a lot of VDD pins, but a few VSS pins. You'll be limited. At the end, if the chip doesn't get enough power, then it won't operate properly. Then all bets are off.
If the physical designers at Intel built in some margin during the power estimation step of the design, then it may be OK.
My suggestion is to get it replaced.