Sometimes you want to upvote a story but you don't want to save it. And sometimes you want to save a story without upvoting it. As it is every story you upvote is saved which can make it hard to find the few you would like to have favorited. If you use the saved stories as your favorites, which I do, you end up not upvoting stories which deserve it just to keep your "favorites" more manageable.
It would allow you to "save" something without requiring you to upvote it. Personally, I would find this very valuable. I often upvote things solely because the title looks vaguely interesting, just to "save" them - even though the item may not turn out to be worth of an upvote.
The 'save' side-effect provides an incentive to upvote which, in turn, improves the quality of the site for all. Separating save and upvote would break this.
