The upside to not including such a thing seems pretty low. Maybe people save a few seconds not reading it? The downside seems quite high if you actually want people to understand your expectations about working hours. The signature may not mean much to someone who has been working for a long time but it could matter more for someone who is just starting their first job, or who has come from a quite different working environment, for example.

I guess one thing you might say is ‘why is this manager sending emails at such times’ but I think lots of people like the flexibility of working strange hours, eg maybe they tend to wake up very early, or want to fit their work-schedule around some childcare obligations like breakfast or a school run.