That's one way advertisers can gain value from using a character. Exclusivity strengthens the association between the character and the product being advertised. Without exclusivity, many advertising characters probably would indeed be of little value and advertisers would probably not use them. I think, though, that this would mostly apply to characters that were created for advertising, or that were outside characters that were not very significant to the public.

If the Michelin Man were used to pitch donuts, for example, I'd find it weird, because I only associate the Michelin Man with Michelin tires.

However, for characters that had significant public interest before they were used in advertising, I don't think exclusivity is very important. For instance, Michael Phelps (yes, I know he is a real person, not a character, but that doesn't change the point) has done ads for Subway Sandwiches and for Speedo, and I don't think either diminishes the other.