Without the valve it will take forever to warm up and then will overcool while driving on highway or at speed.
I think your mechanic thinks that an engine should be easy to touch when warm.. 
My man,,, toyota are the absolute beat when it comes to making a good engine. If they installed it at the factory, trust them… they are much much more informed than your overhaul mechanic.
Btw, if its running on plain water instead of coolant then immediately change it to proper diesel rated stuff like havoline or any diesel rated coolant like the old mercedes benz pale yellow formula or the new blue one, or use CAT ELC (red)
If you use plain water, it will erode the headgasket at the head and block face and also eat the waterpump housing. Its the nature of diesel engines to send shockwaves of combustion events which then cause cavitation in the water which is highly damaging. Proper coolants for diesels prevent this (also make sure your radiator cap is holding pressure or you will be tearing down the engine soon)
