Carrera is bit like the poor man's Jose Reyes. Back when Jose Reyes was here, you knew that if you added everything up he was providing value but it was hard to reconcile that with the sloppy plays you saw on the field. Tulo on the other hand is a player with his drawbacks (mainly the contract) but his mistake free game inspires confidence and makes you feel good about the team and it's really, really easy to like him a whole lot more than what the actual dollar and cents difference between the two would dictate.
Over a full season, Carrera would be about a 1.5 WIN player which basically means below average but well above replacement level which is pretty much exactly what you'd expect from a bench player but if you could trade him for a slightly more expensive player who provides the same value without as many mistakes, as a fan, you'd make that switch in a heartbeat.
I wonder if you could put a number on morale destroying plays? Like surely that has some negative value beyond that of the actual play right?