The mains frequency is an interesting topic. While it is true that it floats still its daily average is purposefully kept at quartz-like stability so that clocks can use it for synchronization. It also needs precise phase synchronization across the whole network (otherwise generators would blow up.)
I'm not so sure if the daily average is kept so stable actually (or maybe it depends on the country).
I worked at a power grid a long time ago, and I recall that in days where the grid had undersupply issues due to climate and high demand, the frequency was kept stable at 49.5-49.6 Hz for weeks or so.
The clock-sync is a nice wishlist feature; but the frequency affects power consumption which often may be more important or financially valuable.
In some of the power grids in the US, the clock-sync thing was something that the operations were required to maintain over a 24 hour period. One technique used to monitor this was a simple electric clock. Electric clocks have synchronous motors, meaning that changes in frequency of the supply would change the time. The target was to be sure that at, say, 6pm, that the clock would show exactly 6pm, even though it would vary a bit during the day.
