[dingocooke]

Dave,
Its not about ambient temp, its about maintaining a constant block temperature; imagine your block if the temp is not maintained at a constant temp by a stat; the temp will be fluctuating depending on driving conditions, meaning the bore will be constantly changing size and shape, and the power critical piston to bore clearance will vary quite dramatically.
When the block is too cool, the clearance will be too tight and piston and bore wear will increase rapidly.
If you use you cob all year round, the engine will warm up quicker on cold days with a stat fitted, and as a bonus your heater (if ftted) will warm up quicker and run at a more constant temp.
You should also notice a more consistant tickover.
It's a massive false economy not to fit a thermostat.