xfce4-session: xfwm4 removed from session, not started on login

[Matt X]

[...]
> 2) Is there any way to ensure that this never happens again? I'm
> thinking that xfwm4 (and perhaps some others) is so important that it
> should always be started regardless of the previous session contents,
> unless the user has explicitly configured an alternative session
> manager.
Please reassure me:
- Would this cause problems if a person didn't have xfwm4 installed, and specified another WM in xfce's Session and Startup?  On one computer, I used Metacity with XFCE for a while (I use xfwm4 now).  On another, I still use Openbox (and will for the foreseeable future).
- Would this cause problems or slow down log in if a person _did_ have xfwm4 installed, but had another WM specified in xfce's Session and Startup?
- How would xfce-session know that one of the autostart applications specified is a different WM?
[...]
-Matt