ST-E2 has a focus assist beam, so in one shot mode it can allow any Canon camera focus in complete darkness so long as the subject is within range of the focus assist beam.
Maybe. I've shot my 1D3 + 85L in conditions of ISO 3200, f/1.2 and 1/80 which is quite dark. My 5D would not do it even with the center point.
And worse lenses for AF like the 50/1.4 which is still otherwise fast enough for very dark shooting make the situation even worse. Even my 1D3 can struggle with that lens when I'm at ISO3200, f/1.4 and 1/60 or so (which is handholdable).
Fortunately I've never had to or felt like taking a shot under such conditions but I do acknowledge that sometimes a lens/camera combo that can do that would be useful, for some.
I do have a flash, whether for the assist or the the additional light, so I'm covered if I find myself in that situation.