ed/ex is well worth learning. The other vi commands are less so, unless you prefer to use vim as your main editor.
If you are a vim power user, finding yourself using traditional BSD vi is likely to be an unpleasant experience, making the vim is great because vi is installed everywhere weak as an argument.
Let's look at a typical novice's session with the mighty ed:
golem$ ed
?
help
?
?
?
quit
?
exit
?
bye
?
hello?
?
eat flaming death
?
^C
?
^C
?
^D
?
---
Note the consistent user interface and error reportage. Ed is generous enough to flag errors, yet prudent enough not to overwhelm the novice with verbosity.
Not really. VIM has vi as subset, and plain vanilla vi is still quite powerful text editor. It doesn't have a lot of bells and whistles, and I would not choose to use it as the primary editor for all tasks, but if I'm on a system where I have to edit something using plain vi, I won't have a hard time doing it and doing it fast.
No, because I know I'm using plain vi :D. I spent a good deal of my life working with plain vi, and the company I work for makes a port of POSIX APIs, UNIX shells and all typical command line utilities for Windows, which includes vanilla vi. So I'm used to it.
I use VIM with only a few extensions (taglist, command-t and snipMate). I also use ctags and cscope, but support for those is built into vim. I really try not to get too dependent on extensions. They are more like convenience rather than something you don't know how to do without. Command+T for example is useful, but you still should know how to get to your files from the command line.
If you are a vim power user, finding yourself using traditional BSD vi is likely to be an unpleasant experience, making the vim is great because vi is installed everywhere weak as an argument.