I guess a better question would be whether the interface is a factor in retaining customers at Lastpass. Clearly, their showpiece isn't the web interface--it's the ubiquity through all their browser extensions and apps so that you rarely need to go through said interface.
At the same time, I agree with other posters. It's low-density to the nth degree. I go to the lastpass web interface because I need to quickly sift through hundreds of websites to find a single password. I don't want to spend my time scrolling and waiting for animations. It looks pretty, in that it looks like 90% of websites shown on HN. That doesn't mean its all that suitable for the task at hand.