I think it's going to be one of those things that gets perpetually argued over. I base my own point of view on what I have seen myself as a road professional as I drive at night for a living ( in urban, rural and suburban settings ). I've seen cyclists using many types of rear lamps and mounted and used in both flash and steady settings. From my point of view the lamps that provide the best in safety awareness are the ones that alert the motorist of the cyclist's presence sooner rather than later. That said there are a variety of ways to achieve that goal.

I see no problem using steady mode but if the lamp is mediocre in brightness there is a chance it will not draw enough attention when cars are near. Secondly, even more chance it will not be seen at distance as there is more chance a dim lamp will blend into the background of ambient light if there are lots of other light sources near it. I've seen this effect myself many times so I know it's true. On many occasions I've seen cyclists using small rear lamps ( on steady ) with likely only about 5 lumen of output. Personally, I'd rather the person use a cheap reflectorized vest than a single shoddy low output lamp.

I've come to the conclusion that if you want to be seen at night as a cyclist you need to "stand out" from the crowd ( or your surroundings to be more accurate ). The more "unique" or attention getting your set-up the more chance you have of drawing the attention of approaching motorists. I know from personal experience that I will almost always have my attention drawn to a bright flashing rear light sooner than a bright steady rear light. Of course if the light source is steady but very, very bright that too tends to draw my attention. We've all heard the expression, "It's the squeaky wheel that gets the grease"....Well, in the world of road night bike riding, the equivalent statement would be, " That which draws the most visual attention is that which is going to be seen sooner rather than later".

Yes, a very bright rear lamp on steady mode will get seen but IMO a very bright rear lamp using a good flash mode will draw more attention sooner. Of course if you use multiple lamps you don't have to use all on flash. More lamps used to create a larger visual awareness is always going to be the better way to go. Sadly, not many people are willing to take that approach.

Summing up, if I'm only using one rear lamp I want at least 60 lumen output and I want it on flash and pointed straight back. Then I'll insist upon some good reflective clothing/shoes. If using two lamps one can be steady and one on flash. No matter what set-up I use I want at least one lamp to be very bright and on flash. Reflective clothing I feel should be mandatory no matter what lamps you use. The bigger the visual footprint you present at night the safer you should be.