Well yes getting all seasons does not makes sense but Xbmc is really stupid on season thumbs
Since the thumb can't be calculated because of language translations (ie the thumb value change if you change xbmc language ....) it's very hard to get them, so the GetSeasons is great for that and creating seasons from episodes does not work
For full datasync for off-line browsing getting allseasons of all show in one query is interesting for remote writers.
About performances for example for audio songs I only get : DetailsSong.ALBUM, DetailsSong.TRACK, DetailsSong.ALBUMID, DetailsSong.ARTISTID, DetailsSong.ALBUMARTIST, DetailsSong.DURATION, DetailsSong.THUMBNAIL, DetailsSong.GENRE, DetailsSong.TITLE, DetailsSong.ARTIST, DetailsSong.YEAR those should not slow the query.
But on some Xbmc it can takes more than 30sec to get the result for 20k songs. (And I really mean 30 secs to get the first { not to download the data).