It is necessary to specify power pins in real designs (as opposed to mere representations of them). This even can be a bit cryptic, and prone to tripping people up.
Most EDA packages have a dialog where you can specify hidden pins tying to named nets. This doesn't show on the schematic and you need to remember to look at it, when you have a question about where it's connected.
It's still usually a no-brainer, but there can be times when it's an important question -- and one that can easily be gotten very wrong.
Consider a circuit using CD4000 logic at 12V for analog interface, that has a digital core at 3.3 or 5V, using 74HC or the like (and maybe an MCU and some other interface junk). You don't want to accidentally run one of the CD4000s at 5V -- its output level won't be logic compatible, and its inputs will draw power via ESD diodes; and you definitely don't want to wire a 74HC to 12V, it'll explode!
So it is a good idea to have some pins visible on the schematic, or if not, then if you are reading the EDA files, to be mindful to check the hidden connections.
Personally, I like the look of hidden pins, but also like showing power pins to the gates. So I take the extra time to hide pins on parts that aren't using them, which requires me to go into the dialog and hide the pins and connect them to the proper nets (even though the one part with the pins visible, is already connected; some EDA packages do this differently, and automatically connect all the parts to the same net, which is a lot less hassle).
I mostly do analog and mixed signal, so it's important to know where the pins are; as mentioned above, a purely digital circuit is only going to have one supply anyway, so noting the power pins every time would probably add a lot more clutter than is worth.
Tim