I've been running with a thermostat locked in the open position on my 21. Even in the summer (25+ºC), the engine hardly warms up higher than 60ºC when driving on the road.

Once I installed a new one, I can confirm that the engine operation is nicer. I guess we all have noticed at some time how engines behave better when well warmed as compared to 'cold'.

On top of this, there are quite a few comments on the forum wrt exhaust bridge cracking on cold engines...

You mention that you'd only run on warm days but, can you guarantee yourself that the bike will never be used on a cold day?

So I'd definitely use a thermostat. It's not expensive nor difficult to install.

Hope this helps.
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