Testing and debugging issues that come up with a specific skin is - which is not the one used by the developer - is very difficult and time consuming as these query params have to be added (typed by hand) to the URL on each navigation.
Changing one's setting in preferences is also an extra 2 page loads and 4-5 clicks away. This wastes too much time compared to editing a cookie in DevTools, which is already open when debugging.
When working on multiple skins or testing both legacy Vector and modern Vector, the time - and focus - loss becomes more emphasized.
Developers should be able to set cookies useskin= and useskinversion= by hand in DevTools. These take effect only when logged in to avoid any issues with caching a page with the non-default skin.