- It's redundant with the indicator badges; if the badges are not adequately conveying this information on their own, this needs to be fixed there. And given T181142, the badges almost certainly aren't conveying this in most skins currently.
- It doesn't work in any skins which use a dropdown menu or otherwise hide the bulk of the user tools by default (which is most of them these days), or in skins that have the user tools on a dark background; for the former, it's not actually visible, so it doesn't actually tell the user anything until they open the menu for some other reason and just happen upon it, and for the latter it can very easily become a case of 'ow my eyes' or even be outright unreadable due to the colours, so that's best avoided as well.
Typical hidden in menu situation:
Coloured header 'ow my eyes' situation: