I used to insist on buying cameras that used rechargeable AA batteries. Replacements are universally available, relatively cheap, and it's easy to take along several spare sets for extended trips.
In contrast, I used to not like proprietary batteries, because (1) the expense and difficulty of getting spares; and (2) if the battery quit, but no replacement could be found, the entre camera would be wasted.
No longer. I've changed my mind, because proprietary batteries:
1. reduce confusion for the neophyte. Need to recharge the battery? Just plug the entire camera to the charging cord. The battery never need come out of the camera. In contrast, buying AA-based cameras means also buying a trickle-capable battery recharger and spare AAs -- which can cost as much as the second proprietary battery.
2. last longer between charges than do AAs -- roughly twice as long, in my estimation.
3. last the lifetime of the camera. We used to think of the camera purchase as a lifelong investment. No longer. With the low price of digital shooters and low manufacturing quality, by the time the proprietary battery wears out, the camera is ready to be replaced.
4. allow for slimmer cameras. Simmer cameras are easier to take with you, and so you're more likely to capture those really special pictures.
And the most important reason of all:
5. reduced consumer confusion. Remembering to swap AA batteries, having spares on hand, not using alkaline, knowing which kind of charger to buy, remembering to have spares in the charger, having to open and close the battery compartment, inserting the AAs upside down -- it is just too much hassle. With the proprietary battery kept inside and charged by the camera, there is none of the hassle associated with AAs.
Don't mind me being late getting over AAs. I was the last on the block to buy a CD player.