I think the goal is to differentiate "cover art" which MC show as "thumbnails" when browsing your collection from actual album art (which can be very large for archival purposes).
So yes for music I have the specific album art images in the same folder as the music files are (Front, Back, CD/Label, Gatefold, Booklet, Inlays etc). These files are also imported as library items and tagged to belong to specific album and so tied to album's songs. However from MC's library point of view I have cover images for every album in centralized folder to which library have always access when it has access to library itself. Also then library doesn't need to handle hires front covers file sized e.g. 20MB when I'm browsing my collection. There's thumbnail caching to partly handle that but I'm not sure what is the size when MC switches to use actual images instead of cache.
For videos this isn't possible at all. Location is always next to file. Generic poster or episode image got from TMDb seems more library related to me (series and season images already are centralized as well as artist images). If I have actual album art files for the release I would handle it like with audio as actual library items (or embed it to file itself in case of matroska but then MC cannot see these at all).
At least option (like with audio already) to do this would be nice.
Also related to file locactions, IMO tooltip image location would be better suited inside appdata than its current location inside MC program install folder.