This is a very interesting post with some valid points, but I have to say I disagree with the statement that Twitter is not a viable marketing platform. It's definitely possible to see diminishing returns with the growth of your follower and following base, but what about other 90+% of brands that do not have that many? What about the brands with 100-500 active followers, who strategically established their connections based on mutual like of content or complimentary offerings, rather than those who mass or mechanically friended people?
The fantastic thing about Twitter (and other social networks) is that it only works as well as one has established, expanded and refined it. If a brand is not seeing results from it, it's time to revisit the strategy and make some adjustments. Remove the connections that aren't active, target the ones that are, and reciprocate the engagement that is already occuring.
From my experience one can absolutely use Twitter as a viable (not to mention free) marketing platform. You just have to have a strategy in place with the dedication to build and nuture your connections, post relevant content that appeals to them and make sure that content gives them a path back to your website should they chose to take it.