This pro-Obama ad serves as an in-case-you-missed-it reintroduction of Mitt Romney's tax returns into the political debate, just in time for Romney's return stateside.
I don't blame them for remounting this issue. It's a solid one, and Romney seems terrified to release his taxes, so the tax returns are basically an endless source of attack ads for the Obama campaign. Of course, they also run the risk of exhausting the media's patience on this topic. Our media outlets are famous for letting go of stories that appear to have no resolution (look at how quickly the news networks rolled over for the Bush campaign during the drawn-out aftermath of the 2000 election, for example) and so the Obama campaign runs the risk of obsessing on a topic that nobody will report.
But the Democrats always seem to find new ways to make the story fresh. Case in point: Harry Reid's comments today about Romney not paying taxes for a solid decade, which may seem irresponsible but which get everyone talking about Romney's taxes again. Someone should start a Romney tax rumor generator. It would get a ton of hits.