The thumbnail size slider serves a similar function in thumbnails mode and (like Naki) I'm not really sure why it would be useful to know the exact zoom level in details mode.
Unlike a web browser, where every page uses different fonts and layouts, everything is a consistent size in details mode so it's pretty easy to tell when the zoom level isn't 100% (and I can't see why anyone would care if the fonts were 150% vs 175%, only that the size was or wasn't the normal 100%). Resetting to 100% when you notice (or if you're just not sure) it's set to something else can already be done via buttons or hotkeys. Opus also doesn't save zoom level changes so even an accidental ctrl+wheel spin will only have a very limited effect. So I wonder what this is for and why so many people want it? Have I missed a situation where it's really useful?
It's more of an OCD thing - it never "feels right" when you un-zoom manually without way to verify it's really back to 100%. I always open a new tab and close old one to reset altered zoom level. It would be cool if some notification/tooltip with zoom percentage would fade-in (and fade-out shortly afterwards) on zoom change. That would enhance user experience