And another issue, maybe not so important, but serious for some: portability. If you travel on business and rent a car, you are usually stuck using the cash lanes on local toll roads. Even if you have a working transponder at home which is compatible with the local system, you aren't supposed to try using it. The transponder is only valid for use in a registered vehicle, which your rental car isn't.
Dave Close, Compata, Costa Mesa CA "Politics is the business of getting firstname.lastname@example.org, +1 714 434 7359 power and privilege without email@example.com possessing merit." - P. J. O'Rourke