N'dou has always been a journeyman, a good fighter who will give an up and comer a good workout or give a top fighter a solid 12 rounds like he did with Cotto. Just falls short of being a top 5 fighter.
Castillo will probably be doing the same after the Hatton fight!
The journeyman fighter will always be in demand for the fact that they are no easy pickings, trainers don't want their potential champions to KO everyone in 1 or 2 rounds, they need to get rounds behind them in preparation for future tough twelve rounders against the elite competition.
I think Cotto has been handled brilliantly in his career and has now got that experience by taking on good credentialed performers who have given him good fights.