It's not really safer. MAYBE you save read/write head wear and tear on your HD because it's not parking the head each time you spin down and spin up. (head wear occurs mainly during those times - kinda like a car engine - most wear on startup) But unless your computer is active, it will spin down the dives even when on.
Anyhow, while saving some systems from boot cycles, you're greatly increasing the likelihood of experiencing voltage spikes, power outages, and dust / debris collection inside the computer - which in itself can lead to failures.
Also things like a backlight have limited life-spans, and will degrade if the monitor isn't powered down (or "slept"). As well as anything that moves (fans, drives ect...)
EVERY COMPONENT wears / ages with use - but you have to be aware that most computers will be pretty obsolete by the time your solid state components will start dyeing off.