Hi,
Currently, the pure-u-1 full width unit is redefined as "display: block". This is done in the section of code that sets up the widths of units, so I assume this was seen as a way of ensuring that it is 100% width.

Why not just leave it as inline-block and set the width to 100%, which would be consistent with the other pure-u-#-# units?

I've encountered at least one problem with pure-u-1 (I don't have time to diagnose in detail because I have a workaround below -- margins of enclosed markup doesn't work as expected), and I imagine there are more. The other css config that is used to set up the units for proper behavior maybe are interacting badly with display: block.
My workaround is to simply add this to my css, loaded after Pure:

.pure-u-1 {
    display: inline-block;
    width: 100%;
}

With this, all is good, but it is a source of layout bugs that should be squashed.