Local code where I live requires an ET with a PRV. I don't know if that is or has become a national code thing, but it's prudent as the EPA has been pushing check valves to protect the water supply, and with one, you'll have a closed system and are likely to put undue stress on your plumbing. Things like washing machine hoses, flexible faucet supply lines or dishwasher ones just don't like being over pressurized over a long period. Unless there's a leak in your system to relieve the expansion (hoses will tend to balloon some, but not enough), your T&P valve on the water heater will open after using a moderate amount of hot water as it tries to heat up the incoming, denser cold water. The rigid pipes won't expand under the pressure from your supply, so any expansion can immediately raise the pressure - the water doesn't compress and the pipes won't expand...recipe for problems. The T&P valve on the WH generally is designed to open at 150psi...way more than your normal home water pressure and nearly double the maximum recommended. With an ET, the pressure should not rise much, if at all, when it is working properly.