Are you saying that if a loop on the sunny side gets warm, its valve will close, so the heat won't be distributed (and, even worse, the temp in that zone will continue to rise because the fluid can't get out of the loop?) I guess that's potentially an issue if you have per-loop/zone control.
With a smart enough controller (ie, not your usual thermostat), you could actually control the valve on the hot zone to open wider in this situation, so that the heat is shared/dumped more quickly with/to the other zones. You could also potentially do some clever things like averaging across zones to see whether you actually need more heat or just need time for the hot zone to blend with the rest.
I must say though, if you have enough insulation under your floor, single zone control is a lot easier!