posted by Cisco's Mom:I understand there is more than one type of chip so more than one type of scanner is needed. Not always practical. >>>>>>

This is very true. In CA. and I assume other states this
problems has improved in the last few years.
My vet originally refused to invest in scanning and chipping for the above reason.
But when our local shelters starting using sometype of universal scanner, my vet started chipping.

I believe hospitals/research labs in CA. (not sure about
other states) are required to look for a tatoo and also
scan a dog for a chip before purchasing a dog for research etc.
(Many dogs were/or are stolen to sell to research)
As for being stollen and sold to a private party,
I guess having more than one form of ID is a good idea.