Even if this is true[1], it never hurts to consciously limit the places where information flows out of your system. The idea is that securing one point is easier than securing n points.

[1] I don't doubt the flow of information, but the rate at which that information can be leaked through a signup page can be severely limited compared to a password recovery page.

but when signing up there is a captcha, unlike the login page.(Okay, unless it's AJAX).

Most websites out there don't actually require a captcha for signup. Paypal certainly doesn't. (I'm not going to create a fake wepay account to check if/when they do email uniqueness validation.)