Fine in theory, however in a tour/group ride situation if you've got one rider substantially slower than the rest of the group it really does put a strain on things socially. Say you want to press on because of weather closing in you can't. If you get to a nice bit of road you want to let rip on you either leave him behind or wait for them etc. Basically he either needs to up his speed or bow out because he could very easily wreck the riding for the, much, stronger riders.
Also this is a very different senario to having a mixed ability group where a large number of the riders aren't that strong & others are very strong.