I'm willing to admit that I might be the fool here, but all vehicles I have owned, I have done the basic maintenance myself and taken them back to a dealer for more involved service. Now, a 20 year old vehicle may not have anything in it which would require the expertise, or rather the equipment that a dealer must have to service their brand's vehicles, however the newer the vehicle the more computer systems and sensors there are on the system. This means that whoever services a vehicle has to have diagnostic equipment.

With OBD and OBDII, a well healed independent mechanic can get the equipment to scan for codes and start a diagnosis that way, however manufacturer diagnostic equipment usually can go many steps further. A basic code reader also requires referencing a code book for that manufacturer to get an on board diagnosis. However, dealer equipment can usually give the mechanic plain English descriptions of problems, and do more advanced engine control and monitoring.

I don't know too many independents that have the money or the resources for the best of the third-party diagnostic systems which offer the same capabilities as dealer/manufacturer hardware.

So, after all those words, it comes down to training, documentation and equipment. The Prius right now requires a Toyota dealer to properly diagnose and service these cars, beyond normal maintenance. As time goes on, this may change and improve and independent shops will gain more capabilities to service these vehicles, but that remains to be seen.

It has been said:
Hybrid drivers come in 3 flavors, greenie, techie and cheapie. Pick any 2.
2005 Prius, Melinium Silver over gray, package 5 (AI)