First, let’s make sure we are talking about the same issue. When I talk about creating multiple websites I mean having example.com for desktops, having m.example.com for mobile users, and example.tv for tv-based browsers like apple tv and google tv . Additionally, you could also have separate domains, sub domains, or folders for flash /non-flash content. The first problem is this creates a huge maintenance point. Unless you have the staff and budget, maintaining multiple versions of the same website is going to consume a larger and larger amount of resources. The more pages you have, the more versions you will have to maintain, and it will grow exponentially.
Full article: http://www.wolf-howl.com/seo/dangers-multiple-website/