found the quote from Adler (not Squyres)
http://planetary.org/news/2004/mer-update_0923.html
"So when will the team call it a day for the two intrepid explorers?
'If the rovers stop driving, if they can't roll, that's pretty much the end of the mission,' surmised Adler. 'The whole point of these rovers is to explore new places and once they can't do that anymore I think it's probably not worth funding operations."