YES! E.g. try running emacs on $host and run bbdb; go to some other machine and ssh ~/.bbdb $host:~/.bbdb; then ssh $host emacsclient. Now it's unresponsive until you walk over on your feet to $host and answer the revert-or-not question :(
You can use both in the same file; you just need to declare the dynamic ones using defvar, etc. If you turn lexical-binding on blindly, the compiler will usually tell you which variables you need to mark as dynamic.
It has to be this way to remain backward-compatible. Similar to the issues Perl is going through with things like enabling strict by default (you have to request at least a certain version, I think 5.12), and distinguishing a Perl 6 file from Perl 5 one.
