Trisha - the one exception is if Carter is planning to do a info site i.e. "Restaurants in Mexico City" or something like that ... you could develop a suite of sites like that that don't need to be updated as often and are designed for adsense i.e. 100 sites making $1 / day is $100 day
Generally though, I agree with you that it is a good idea to keep one site, grow it and then you also have only one site to promote.
Travel WRITING is only one very small part of the equation - promotion is almost equally important and often more so.
Again though, it all comes back to motivation ... are you looking for free stuff, make some money with adsense, or grow your resume / clips list.