I only recently turned standby mode on for the PS3 (not the PS4) in the interest of playing with remote play via the PS Vita. I did this back when I bought a PSP and promptly stopped using it, but I figured why not try again.
What I noticed with the PS3 is that the little green activity light stays on and the fan spins at the lowest speed. I couldn't figure out what that noise was in my living room when nothing else was on and it turns out it was the PS3. I didn't bother plugging it into the WattsUp to see how much more current it was drawing in sleep mode and ended up just shutting off Remote On. I never take the PSV anywhere anyway.
On the PS4, I think it's mostly useful for automatic updates, charging a controller (which the PS3 couldn't do in standby), and leaving off from where you stopped before going into standby, which I generally don't do and strikes me as being kind of lazy, but everyone has their own habits. If I'm done playing a session, I'll shut the console off.
I normally don't use standby mode on the PS4; just when it needs to be in standby.
It seems to take just as much time to wake from standby mode as it does to start up from power off mode, so no time saved there.