I don't think DATE can always be substituted for YEAR because it forces a degree of precision that often is not known or knowable, therefore not correct.
There are many times when when the need is to tag something ONLY at the YEAR level.
For instance, it is standard for music, TV and movies to be identified by the YEAR of release, but often not the month and rarely the day of the month. So the ability to specify just the 4-digit YEAR is mandatory. Fortunately, this seems to be possible by using the YEAR field.
However, there's a side-effect that can be a problem. Apparently, MC actually assigns a full date when only the YEAR is entered. It seems to default to 01-01-YEAR. Perhaps this is an understandable internal need, but it bolts-in an incorrect date, and thereafter, the only way to know it's not the real full date is to surmise based on it being the "odd" date of 01-01-.
This hasn't bothered me with music, TV and movies because I would rarely if ever know or care about anything but the YEAR.
But for tagging photos, it is already getting crazy. It's common to have only a partial (year-only, or year-month-only) date for an old photo. If I have a picture of my grandparents on a summer adventure and I'm pretty sure they always did that in July and I know this photo was 1960, I'd like to tag it as July 1960, and not have MC force it to be July 1, 1960 which forever would be a wrong fact about the photo. But partial dates don't seem possible, probably because there is date validation built into date fields.
One workaround I'm considering is to create custom fields for year and month (but it has to be different names like YR and MO) and not identify these to MC as date fields.