One thing to remember is that pigmens are particles and must be suspended in a liquid carrier. Many of us with LF printers are all too familiar with what can happen when carts sit for prolonged periods without agitation. The expiration dates, I would assume are related to the ability of the particles to remain in suspension.
In this regard, I spoke to Sr tech reps from both Canon and HP in recent weeks precisely about the issue of pigment settling. Both the IPF (canon) and Z (HP) series printers include maintenance routines that periodically agitate the inks in the carts. Both companies, and presumably Epson design their formulations to remain in suspension over a given period of time. I can't yet shake the habit of manually agitating carts every couple of weeks or so, so can't comment on how long before settling occurs in the new printers. But if it is indeed safe to wean myself from this preventative measure, that's an important advance that gets little press.