I run my light in flash during the day because I ride in urban environments where I'm constantly transitioning between shadow and light. Even with light colored cothing, a bicycle is easily lost in the shadow of a tree or building.

During the summers, it gets very foggy here in the morning so I am starting to run my steady light also during the morning commute. It gets bad enough here that visibility is only about 30m sometimes.

Besides this, I use rechargeable batteries. The cost of the lights and batteries are sunk. The ongoing cost for electricity to run the lights is something like 1/4th of one cent per week for the amount I use my light. Since there is almost no cost and no harm in running the lights during the day, why the heck not. The batteries will last maybe 10-12 days in the back light and around 30 days in the front and I recharge every Sunday so my batteries have plenty of capacity in reserve unless my riding goes up a lot.

My 18650 light, though, only has about 2 hours run time so I have to charge that maybe every other day if I use it daily.