Variable supply water temperature is a benefit in the Seattle area. Should you also have A/C, which I'm sure you do there can be a big savings for such a small control in both energy used and equipment life.
Part of the Seattle weather problem that a good part of the other country does not have is that heat is required in the evenings and night with cooling required around noon to two in the afternoon. Then back to heating again in a few short hours.
This can be amplified by too hot of water in the circuilating system of the home. The heat that is in the floors is basically a heated mass that takes many hours before that mass will cool down to the point of a normal heat requirement within the conditioned space of the home.
So to off-set the "over heating" by the thermal mass the automatic controls will energize the cooling system(s) to compensate for the over heating. In essence, you are using the air conditiong to remove the heat from the house.
Having a "hot water reset" on you floor system is a wise thing due to the extremes in temperture that Seattle experiences in the Spring and Fall.
I have homes that require heat in the floors when a portion of the home is located in the shade of the north part of the home while requiring cooling in the south part of the home.
There can also be a situation where the water loop is so hot that it effects the thermostat before the rest of the floor gets warm causing cold spots and drafts throughout the home. The thermostat is typically mounted on an inside wall which is where the feeders for the floor heat is. So the inside core of the home can become warm, shutting off the heat as the thermostat is located in the center core which leaves the outer spaces of the home cold compared to the inner core.
Something to think about.
"The American Republic will endure until the day Congress discovers it can bribe the public with the public's own money.
- Alexis de Toqueville, 1835