Seems the reason for the disclaimer is that there are ways to break out of the open_basedir rule. I would still use it on a shared host but don't count on it as your only security. Also have each virtual host owned by a different user and run the apache process under that user account for the scripts on that host.
To your more general question though, I do think the age of shared hosting is almost over. Virtual host technology has advanced to the point that a shared host is almost useless.