Let's back up a bit here. A lot of people seem to have accepted the argument that Cam's offense does not use a 3rd WR to any significant effect and that the FB position is more important in that offense. That argument isn't just true on its face. In 2009, Kelley Washington had 34 catches out of the #3 WR spot and was very productive when he was on the field. In 2010 a broken down TJ Houshmandzadeh had 30 catches out of the #3 WR spot and was fairly productive despite being a shell of himself.
In 2011, we DID use a #3 WR, only he wasn't listed as a WR on the depth chart. His name is Dennis Pitta, and he had 54 catches while lining up at slot WR 70% of the time he was on the field. So the argument that "Cam won't use a 3rd WR anyway" is just plain wrong--I think Cam is one of the worst OCs in the NFL, but don't level charges against him that are just patently false. He has used a 3rd WR when that player has been an adequate contributor.
This offense still uses a FB more than any other team, but all that means is that the FB is on the field 50% of the time instead of 30% of the time. The playing-time divide between the FB and the #3 WR (or #2 TE as the case may be) is probably a lot closer than you think.
And by the way, I'm not sure the 11 million the Ravens spent on Leach would have been best spent on a WR anyway. I think that money would have been better spent on OL.
How often do the 49ers use a fullback? I haven't looked into it but I would guess that if there is 1 team left that need a real crushing fullback it would be them.