hide_span.sass

// Mobile-first approach. Usecase is to hide one of columns by default (mobile) 
// and show additional info on desktop (when @media is active)

// !! Working version !! Original problem description is in initial commit.

// @media vars:
$media-desktop: min-width 769px
$media-mobile:  max-width 768px

.page  
  
  +use-grid(100% (1) hide)
    +container
    .sidebar
      display: none 
    .content
      +span(1 omega)

  +breakpoint($media-desktop)
    +use-grid(98% (4 6) 0.1 hide)
      +container
      .sidebar
        display: block
        +span(first 1)
      .content
        +span(last 1)

Two things strike me here:

1. I'm not sure I understand why this is better than display: none; and display: block; - what "doesn't work" about that? IE6 required an inline display, but I'm not sure that's worth worrying about.

Unrelated:

1. When the span is full-width (in your initial grid) we really shouldn't be floating, and setting a width. We can easily detect that and do something different. In most cases, you actually don't need anything - block items are full-width by default. Maybe a new layout context, or a new row? clear: all; could work, but may surprise people when they have to remove it somehow.

Yup, the case was in full-width span.
+span(1) added default gutter of 1/4 which is margin-right: 25%;. And it kept this gutter in grid that overrides the default one.
So if we add omega argument: +span(1 omega) then overriding of it works properly

I think if it's possible to detect that span is taking all of the columns then making omega argument implicit may be a reasonable solution.
Of course your proposition not to float it at all and make use of default full-width of block element is much better. I'm afraid only clear: both; can help to achieve this but as you said it will be more confusing for people to explicitly do clear: none to override it. And adding clear: none into default +span() output will bloat the compiled css too much so it doesn't worth it either.