Right. So it depends on your use case.
utag_main_vid (visitor ID) is how we uniquely identify users.
utag_main_ses_id is a timestamp that identifies the session, but two (or more) users could theoretically have that same time stamp. That's why Ty recommends you use both.
If your tool is correlating a pre-existing visitor ID with this session ID, it should be okay, but if it's expecting a unique value, you will want to join these two values or it might see two users with the same ses_id as being the same person.