Hey dude..
Cells are cheap. Sure, the "don't use after" date may partially be a CYA thing, but there may be something that the manufacturer knows about how they make the cells that I don't, so I'd rather follow their recommendation.
I've definitely seen some weird things with cells. I had one AI cell that worked great up until 90 minutes, then would read basically 25mv low (~0.5PO2) for 30 minutes, then would wake back up at the two hour mark. It was repeatable every single dive, I let it ride for 5 or 6 dives while waiting for a replacement to come in the mail -- and that was a new cell that had this weirdness.
Because of some of those experiences, I'm a firm believer in rotating the cells in such a way that each cell is from a different lot number. Think about how frequently there's a recall on a particular lot of tainted food due to a processing problem, now imagine a batch of O2 cells that had a flaw in the manufacturing process -- they could all potentially "fail" in the same way. I don't want all three of my cells to be from the same batch for this reason.