Two points--one, the lack of changing numbers shows that people already know how they are going to vote. They aren't listening to messages.
Two, elections have changed since 2000. There's a great article floating around explaining how candidates now target very specific audiences, as opposed to the old days when they would target broad regions. The South, for instance, used to be targeted for its conservative, religious, blue-collar, agricultural, and racial voting habits. Now campaigns will target a group of people identified by specific beliefs, or buying practices, or maybe an intertwined mix of factors. Profilers can find people who bought Bibles and Starbucks coffee, for instance, and target a message to them. They can deliver these messages through social media, or even just through public broadcast by tailoring dog whistles to appeal to one group instead of others.
So, when you hear Ryan calling rape another means of conception, he's going after a certain demographic that no longer is defined regionally, and counting on that claim not hurting him with voters he actually has a chance of winning. Both parties do it--the whole corporate-personhood thing is our own dog-whistle, for instance.
So conventions now are meant to fire up the bases with bold position statements and lots of cheerleading, so the base will deliver the cash, that can then be used to target audiences. It also means polls will be less reliable, since the demographic assumptions of the polls may not work.
So while Romney getting no bounce is good in general, it may not matter in a close election.
Here's the article: http://campaignstops.blogs.nytimes.com/2012/09/01/why-campaign-reporters-are-behind-the-curve/