It is sometimes very difficult to answer a "why" question ... so I try the second one:
Create an action of the type "Format tag field" for the field YEAR
Enter as format string:
$left(%year%,4)
This should reduce the length of the entry in YEAR to just 4 ... and if it was already equal to 4 then it won't do any harm.
Regarding this question. I'll answer why i use this personally. Theres DJ software called 'Traktor Pro' which utilsies the year field in the format YYYY-MM-DD OR YYYY-DD-MM. So it shows up in the format YYYY/MM/DD OR YYYY/DD/MM. Also iTunes recognises that as just YYYY. So there's one standpoint for why to use that format.
You might also what to back-up the YEAR tag to a BACKUPYEAR tag before you go deleting valuable data.