Some comments compared riding w/o light to driving a car with headlight off in daylight, but I think that's not a good comparison. Car headlights are very bright, so it is effective in daylight. But most bike lights are not bright enough to make much difference in daylight. (Of course it can make a difference if you use an expensive >2000 lumen light during daytime.)
Something to think about - the daytime running lights used on automobiles are actually LES bright than either the low beam or the high beams. Thats particularly true in Europe where light pollution issues has resulted in very low powered daytime running lights being the norm.
As far as bike lights go, its apparently just as easy to pick out a 300 lumen light as a 900 lumen light in the daytime. Partly because unlike auto headlights, bike lights have no cutoff to shield the emitter. I certaily agree a 50 lumen light wouldn't be easy to pick out during the day - but at night it wouldn't be much better either. I'm currently experimenting with a pair of amber 300 lumen running lights to see if motorists pick out the color easier.