The idea of drafting a player and keeping them on the roster but having them inactive like they did last year with both Lamichael James and AJ Jenkins is in play. It did work out because when injuries struck the 49ers, those two players were ready to step in and contribute (even though AJ Jenkins had more drops than catches... and he only had one drop). But Lattimore's knees are already a ticking time bomb and wearing them out in practice without utilizing them during games would seem a bit silly. I don't think it is realistic to draft Lattimore in anticipation for Gore running out of gas in a year or two, because Lattimore may have to bow out of the league before Gore.
If the 49ers do take a RB, expect it to be in the 6th round or later. Everybody is going to be looking for the next Alfred Morris (6th round) or Darryl Richardson (7th round). Expect it to be a player from a small school that does one thing exceptionally well i.e. lightning quick or Hulk strong. They don't need a RB now, and shouldn't in the near future, so I don't think a guy like Lattimore will be in play.