It's not a great idea, though I run without one. As long as you let it warm properly - which will obviously take longer - and ensure it doesn't get too cold whilst riding you'll be fine. On really cold days you might find you have to tape off a bit of the rad to get it up to temp.
But then, you'd rather be in the pub if it's that cold!
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MC21SP Plaything
BMW F800GS Bumblebee
Triumph 9551 Daytona Big boys toy
FJ1100 Sporting relic
GTS1000 oddball