My guess is this idea is so his company might be in a position to mop up all of the lost and helpless Twitter users when the VCs and investors that pour money into Twitter, suddenly realize that it wasn't worth it after all.
Twitter is really expensive to run and maintain. To provide a service with that much traffic, worldwide, and with the speed it does is a huge operation. As the comments on here prove, nobody is actually willing to pay for the service, and the ads are almost non-existent. So, Twitter has no revenue stream (or not much of one, anyway). Investors invest because they expect a return on their money, and currently Twitter is a bottomless pit that, for some reason, people continue to pour hundreds of millions of dollars into, hoping that someone at Twitter will suddenly figure out how to monetize it. The investors seem to have forgotten the lessons from the first dot com crash, which is basically that just because a free service has lots of users, that fact alone does not make that service 'valuable'.
If the ads get more prominent, they will still be ignored, and there will still be no revenue stream. If the ads reach a point where they are intrusive, people will leave the service. I'm sure that if Twitter vanished tomorrow, nobody would particularly care for more than a few days of moaning on some forums, and then it would be back to a far more productive society. But, there will be a smallish percentage of Twitterers (twits?) who simply can't live without their daily dose of 140 character vacuous and irrelevant bulletins, so where are they going to turn to? Well, if there's a Twitter equivalent with no ads, and no prospect of the service going out of business due to the users actually contributing a little bit of cash (and I mean a little bit) to keep it going, then it may all of a sudden not be such a bad idea.
Twitter cannot continue as it is right now. It simply can't. There cannot be an unlimited supply of rich morons willing to throw vast sums of money into something that will never generate revenue in its current form. Well, if there are then I'd like to meet some of them....