The reason why the new code requires an expansion tank on each and every new installation is for the protection of the public, of course.
For if no new expansion tank is installed for that heater and one day a homeowner installs a pressure regulator, the expansion tank will not be there. Not every home needs an expansion tank with there current plumbing system, the code is there to overprotect and to take care of alot of “what ifs” just in case someone alters the system in the future, the homeowner will be protected.
So pay up and protect yourself.
The Local Plumber
Tustin, California http://www.TheLocalPlumber.com