I've been using the primary selection since I installed my very first Linux on a Computer in 1994.
It's so simple: Select text, change window, middle-click, happy
With the latest updates - it started when I first tried out wayland and quickly switched back to X11 because wayland diden't support primary selection - the primary selection bekame more and more flaky.
- It is no longer clear, which text you paste when middle-clicking. Might be a selection you had long forgotten about turns up ...
- Some programs (firefox and thunderbird are the most prominent) don't allow selections to go into the primary buffer at all
- Some programs ignore middle-click (again firefox and thunderbird are the most prominent)
- making use of the clipboard AND the primary selection nowadays is virtually impossible, as you cannot be sure, the clipboard keeps its text when you select another to overwrite it with the clipboards content
there are a lot more annoyances I observed in the last months that have their origin in my habbit to rely on using the clipboard and the primary selection as two distinct operating principles.