I can think of many use cases for a rem and em system over a px system (TV’s and shared component libraries are the two I’ve absolutely HAD to use them for), and because we have CSS pre-processors here in 2k17, it’s become natural to include a standard utility partial that converts pixels to rems and ems as needed.
I think something that is easy to forget when developing is whether or not something needs to be 1) reused in a scope out of your control, or 2) changed in the future in a scope outside of your control.
So while you’re right, there aren’t nearly as many use cases for using a rem and em system over pixels as the internet would have you believe, there are literally no reasons to use exclusively pixels.
If you’re making a static site that you would toss in an agile fire and never use again, never expand with a development team, and aren’t using a CSS pre-processor that can do two simple arithmetic operations— you can safely be lazy and type exactly what you see in a style guide.
If you’re trying to build a more reusable, mutable system with CSS pre-processors; do yourself, and everyone who could use or alter the system in the future, and add a util for converting px to rems and ems.