Good idea in general, but it's not so simple:
- Gwenview has Previous/Next buttons to jump to another image (the history dropdown should be fixed to make them more useful, I guess…)
- Gwenview allows to add multiple tags without having to reopen the dialog

Nevertheless, Dolphin's dialog (which I assume is from baloo-widgets?) also has some nice features:
- Easy way to select and apply existing tags
- Good support for nested tags (type "/")

I'd imagine going forward like this would be best (because I'm very much in favour of unifying the experience everywhere):
- Add adding of multiple tags to Baloo (e.g. instead of "Save/Cancel" use "Add/Close", with "Add" not closing the dialog)
- Port Gwenview to Baloo's improved widget, add back navigation buttons (or find a better way to serve their use case)

+1, sounds like a plan. Definitely in favor of using common widgets here so they don't develop separately and each app can benefit from improvements made to the widget itself.