I don't think it has any valid use case (especially now that it always paints sharp sizes of icons)

If you have more than one panel per screen edge it is very useful to make sure both are the right height. It helps preserve settings across installations, for example you can quickly and easily tell your panel was the same size as before you reinstalled.

This seems really like a constructed use case to me. The vast majority of people will not bother will exact sizes. This doesn't sound like something I'd want to clutter the default Plasma panel with.

The reasoning is not different from Plasma 4.x times.

If showing the exact pixel size clutters the UI, would it be possible to include panel size and position settings into the panel options dropdown (or a separate dialog window). 

Not only would this feature usable for people who like to adjust everything pixel-perfect, but also for users that have problems with drag & drop or laptop users without any mouse.

Due to the inaccuracy of the trackpad on my hardware it is extremely hard if not impossible to size the panel exactly how I want it.  The ability to specify exact pixel size, especially with regards to multiple panels and separate monitors is in my opinion a very defined use-case, and the capability should be a standard feature in Plasma.

I agree that this should be a feature in Plasma. It's very useful to be able to see the height of the panel when resizing, for the reasons provided by the others.

I simply don't get the argument about it being a constructed use-case. I mean sure, the vast majority of people won't bother with exact sizes, but that same majority also won't bother changing the default Plasma layout much beyond what it already is; ergo, a lot of things in Plasma are "clutter". Every other DE allows me to have the same panel size on two different monitors without touching a config file, why shouldn't Plasma.