My usual front light is bright enough to be irritating to me when flashing even on street lit roads. I like cycling to be as pleasurable so I put it on steady. I also don't want other road users irritated too much as it makes them less calm and considerate towards me.
You could make an exception in heavy rain, but to use a very very bright flashing light all the time is overkill and counter-productive, I think. I see lots of this in London now. Horrible on the retinas.