It may sound like a conflation of the other reasons that were listed (just lower the barrier and people will care enough to sign up), but it's more important than that. I have to tell this to my clients (website developers) over and over again - No matter how much you tweak your signup form, the single and best asset you can have in enticing signups is something that the user really, really wants to sign up for.
Put differently - There surely is some wiggle room in pushing a couple of percentage points out of your signup statistics, but that often ranges with the margin of statistical error.
No optimization will ever make up for asking users to sign up for something they don't actually want or need.
While blog posts like this one are nice for people who obsess about optimization, I have seen them do a lot of damage - for instance, I still get people who are convinced that the one thing that is holding back their business is having a four page signup (membership selection, registration details, confirmation, checkout) instead of a three page setup. Some even go so far as to demand that everything should be on a single page.
In my experience, there is a clear, inversely proportional relation between the obsession over signup optimization and the value that the website offers to customers.
If your website gives out free gourmet food and massages, you can literally require people to fill out ten page forms and ask for confirmation via standard mail - your signup form will still convert users like crazy. At the opposite end of that scale, there are forms like the recent April Fools joke 'Google Nigeria' - the simplest possible form: Enter your Credit Card details and be done.
Sorry about the rant, but this (pardon my french) wanking about optimization has produced a lot of endless, annoying discussions with clients who burn up all their time on optimization - time that they should have rather put into having a sound business idea that people would actually care about.
(And yes, I know that #5 and #8 go in the same direction as this rant, but they just range in a completely different category and all this just triggered my rage mode.)