There were so many - loads - of points in DH at which I kept expecting - practically screaming - for Snape to come into it. Not just this scene, but this one would've been pretty Darn Good, IMHO - what with the doe and other implications.
Thing is with the sword, isn't it a "true" Gryffindor that can only wield it - i.e. what defines a Gryffindor, i.e. bravery? Even Harry said Snape was the bravest man he ever knew (though 'bleugh' to that line). So if, as Dumbledore suggested, Snape might have been sorted into Gryffindor at his present age, I assume he's brave (well brave) enough to use the sword as per that definition.
JKR threw away so many facets of Snape' it's beyond belief.