He made a somewhat interesting point around this -- that even if you do hapen to be interested in a trending topic and select it, a bunch of mostly unrelated spamish tweets will be there to greet you when you select the topic.

Sure, but Twitter should be able to fairly reliably detect that someone like Marco isn't interested in the global trends, and serve him stuff more likely to be of interest.

What Twitter's currently doing is akin to Google text ads on HN for Justin Bieber-themed desktop wallpapers - it's completely untargeted.