This alternate ending to I Am Legend is (IMHO) far superior to the abrupt version in the theatrical release, and fits in better with something that happens in the film about midway through.
At one point in the movie when Will Smith's character took the lead zombie's mate away from them, the zombie reacted with extreme rage, indicating that there was some vestige of humanity left in them. But throughout the rest of the movie we never saw any more hint of that, which felt... "unfinished."
Well this clip addresses that very nicely, plus ( ------ SPOILER ALERT ------ ) doesn't have that nihilistic ending with Will Smith dying. I don't know why they didn't go with this ending... maybe they thought it was too cliched for Will Smith to survive - but in this case I think it would only have made the film better.