I personnally wouldn't do it if I wasn't serious about buying in the near future, but that's just me.
As for your other question regarding the old 'sales guy ride along thing'...well, when I test drove a new 2004 F-150 a few months ago, the sales guy just tossed me the keys and asked for a copy of my drivers license. Of course, I guess test driving an F-150 and a Mustang are two different things. So, I dunno.
I betcha if you asked to test drive a Mach 1 or Cobra the owner would probably want to ride along!
E