Where I live, local code addendums have required an ET since the mid-80's, so this is not new. To protect the municipal water system, all utilities are or have installed check valves. So, even if you don't currently need one, it may be time to consider adding one. If you have any plumbing work done, you might just consider having the person include that task in the job list.
Having a closed system would not make a water hammer more or less prevalent if it was properly controlled. The higher the pressure, the faster water can flow, and just like walking or running into a wall, when you try to stop the water flow, the faster you go, the more energy it has, and the more likely you are to get a hammer effect. Notice, though, that that tends to only happen with a valve that closes abruptly (like some toilet fill valves, dishwashers, icemakers, and some other valves and devices).
Without knowing what your water pressure is and what it is doing, it's hard to tell if you might also need a pressure reduction valve in addition to an expansion tank. FWIW, a pressure reduction valve makes it a closed system as well as any check valve that the utility may have installed.
Unless there's a leak somewhere in your system, once you use hot water in a closed system and the tank starts to reheat, the pressure WILL go up if you don't have an ET or there's a leak. Basically, the pipes are not elastic, so any volume increase rapidly raises the pressure...think pushing against a wall...the wall doesn't move, but the pressure rises...same as the water pressure.