If the Saints have a specific kind of back with a specific skill set in mind, and Wells fits that description, and Jenkins is off the board, why is it a bad idea to pick him???? They can't count on the possibility that the guy that they have targeted in the 4th round will be there, unless they have Wells and some other back rated pretty closely, which is almost certainly not the case. All they can go with is what they see right in front of them, which is Wells sitting there at #14.
They definitely seemed prepared to upgrade the RB position, so if RB is the BPA at #14 (in their opinion, not yours) then it makes sense to go with Wells.