Janet,
This has been good news. [I have been using this import for awhile].
I have a couple comments, however:
1. It would be *really* useful for there to be a way to delete and/or
modify these in bulk. One of the most beneficial uses of these is to
'tag' records with these values. [often temporarily].
And the current problem is that once you tag a record, there is no way to
delete or overwrite it. Hence, you'd have , on the contact card, an
ever-growing list of these, rendering the whole section of the contact card
very cluttered and confusing.

2. If I define one of these fields as an enumerated value [e.g. a dropdown
as an entry method; tied to a table], I cannot import it. The import just
fails with no good diagnostic. I've been told this is a limitation. It
seems to me to be some missing functionality and/or a bug.

Thank you.
john

Good news - in early 2019 we added this capability through Data Imports.  Read the details at this link here   under the section Update 'Admin View Only' Fields in Bulk. 

It's been 3 years since first introduced.

Any updates here?

This would be *extremely* useful as I use these fields as a way to temporarily tag records for operations.   e.g. to send email to an arbitrary list of students.  the only alternative would be to manually tag each user.