I've worked for 3 airlines. 2 (both longhaul) had SOP that it was always on. 1 (shorthaul) had no requirement. Working longhaul I want the weather radar on all the time on at least one screen. Pure and simply because when you're regularly flying through the night either over Africa or over the Atlantic you're dog dead tired and no matter how good you think you are you will make mistakes through fatigue.
Trap the error before it becomes one. It doesn't cost you anything it has no ill effects on you and I've found having it on all the time has forced me to learn how to use it properly. It's incredible the number of times I have found embedded CB's where absolutely nothing was forecast.