I looked at expansion tanks today, and in the instructions it plainly states that the tank WILL eventually fail. Given that both of my hot water heaters are inside my home and in places where a flood of water would be destructive and potentially VERY expensive, I think that I could dump a little water every now and then.
Actually the expansion tank ios also used in some homes with an anti-backflow valve. If the water heater is filled with cold water and then heated, the water expands and has to go somewhere. Without a check-valve it goes back into the water main (small amount - but enough to build incredible pressure in a closed system) With check valve it can't get out so an expansion tank is used. A thermal pressure relief valve on the water heater should NOT be depended on to bleed off the pressure due to expansion - what happens if and when the water heater is shut down and the system cools??? Also, the valves tend to leak after a few operating cycles. If the relief valve has been activated it SHOULD be replaced.
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