In the ACP's Member Management, admins can see a list of members, their email addresses, IP addresses, group, and when they joined.

I'd like to suggest one more display column for that list. Namely "Last Seen," which would be useful for checking how active members are.

Attached is a screenshot of what it currently looks like, with my idea added in.

That is a great idea and have often thought the same as well. Would very much like to see this included.

Isn't there a way to customize the ACP member list to your needs via the templates?

Isn't there a way to customize the ACP member list to your needs via the templates?

Yes. It's templated like the front end via the skin_cp files in /applications/members/

Note that modifying the files is not supported.