When I was coming up through UNIX programming, GUI editors were not that common, and certainly not across the Sun, HP and IBM systems I commonly used. On the other hand, vi was always there. It's old-school. It's infrastructure.
I'm going to say that yes, you should learn it if you are taking a Linux/UNIX centered development path. There will be a time when you just need to quickly edit some dumb little text file, maybe over a telnet or ssh connection, and vi will do it.
Google "vi tutorial" and spend some time. It might be fun, as well as a little frustrating ;-). Just remember, when in doubt, pound that Esc key!
A few additional comments:
1) I run vi from the command line. When I'm in a GUI mood I choose something else.
2) Whenever I load a new Linux system, I add the full vim package. Vim is vi plus nice things like colored syntax highlighting. When I can, I vim rather than vi.
3) Back in the old-school days there were "vi versus emacs" arguments (see Editor War). I won't comment on that. I ended up a "vi guy" alternately because I like a light environment, or I just happened to use vi first.