Hybrid vehicles have a significant advantage over PEVs when it comes to battery life, which is that they don't need to utilize the full capacity of their batteries. The control system on a Prius tries to keep its main battery at an average 60% charge, which significantly lengthens its life. That's not such a big deal when you can recharge the battery at any time off of the gas engine.

Depending on heat, a fully charged Li+ battery will lose between 15-30% of its capacity per year. If you keep its charge between 40-60%, it will lose only around 4-10% per year.

Tesla estimates that after 5 years, their battery packs will still have 70% of their original capacity, which would seem to indicate that this is indeed what they're doing (70%^(1/5) = 93%, or 7% loss per year). That sucks for a car which is entirely powered by its battery, since it means that a Tesla could, at a long-term cost to total capacity, potentially have at least twice the range its control system will allow. It seems like that would be a decent feature to give for people making occasional long trips.