Not wanting them to clutter the stream of discussion is a really good reason to remove them though.
Aside from one's profile page, avatars and user titles (left column stuff) is where one shows personality. The right column is designed to be all conversation all the time. Signatures interrupt that.
Additionally, "because they are antiquated" is actually a fairly good reason to dump them. Internet fora are dying all around us to social media sites—whether it be social networks like Facebook, microblogging sites like Twitter, or something entirely different like Reddit. These services all offer a far simpler discussion UI than traditional fora. What was one of the first things this new generation of social platform stripped away? Signature lines—and I don't see anyone on Twitter, Facebook, or Reddit begging for them back.
We can argue all day about whether or not a stripped down UI is actually inheritly better for discussion, but I don't believe a reasonable case can be made claiming that most of our users aren't more familiar with these newer, simpler forms of online discourse. In a day and time when we are often the first forum people have ever used, there is a real value in defulting to a familiar, streamlined presentation that cuts out primarily frivolous elements from days long since past.
Ultimately, the functionality will stay. Anyone who wants it can switch it back on again. That said, I'm pretty sure it won't be the default much longer.
