There is nothing wrong with using a light that runs on "off the shelf" rechargables but the run time will tend to be shorter and it narrows your options a lot. Designing a system around a proprietary battery back allows the manufacturer to maximize run time vs. power and weight (good thing for you) and locks you into buying a new one from them (bad thing for you) should it fail.
That said, even a proprietary battery should last for 2-5 years depending on type, frequency of use, and how you take care of it. For many people the battery will outlast the usefulness of the light (either they will upgrade, quit riding, or lose parts of the light). I don't think it is reasonable to say it's stupid to buy a system with a proprietary battery pack.
It's always a good idea to poke around online to find out what the best way to take care of the type of battery you get is. LiIon batteries are different from NiMH and SLA. If you only use the light during a certain season you should pay particular attention to the directions for storing the battery.
There are a few good (bright, well-made) front lights that will run on the AA or AAA's though, check out Dinotte for starters. Most taillights (including the forum favorite Planet Bike SuperFlash) run on standard rechargables.