Honestly there's no need for a Twitter with porn. It's not like there's any lack of options for publishing or viewing porn on the internet.
Of course if Twitter wants to host porn more power to 'em, it's just not a make or break differentiator. I have a feeling in a year or two people will assume that if you're still on Twitter, it's for the porn, with the plausible deniability of "no man, it's for the free speech!"
Although they did do a very good job. The app is well thought out and the service has been rock solid since launch.