The filers are the people that just organize everything into folder after folder after folder. The pilers are the people that keep everything in their inbox and just rely 100% on search. I actually heard of one person who also creates folders within their deleted items, which is pretty insane. And people spend an enormous amount of time on filing -- not just writing or reading emails, but actually spending up to a couple hours a day managing email into the correct folder.
People today in the corporate world average around 200 emails a day. This rate of email growth is just going up and up. It's not sustainable to manage it folder by folder by folder.
Enterprise search tools are still not quite there yet in terms of how reliable they are. But the tools are getting better, and you can foresee a day quite easily where we'll just rely on search for this.