Removing them from public view does reduce your valid content -vs- noise ratio. And of course it helps that you have less duplicate content (as a signature could repeat hundreds of times).
XenForo has a default option to allow you to hide sigs from guest view.
In addition, there is even an add-on that limits sigs to only 1x per thread (per user)
http://xenforo.com/community/resources/signature-once.179/
In my opinion it does help not to display sigs publicly (or excessively). Not just for SEO, but as a spam deterrent