Every team needs an Osman. Man Utd for instance have Fletcher and O'Shea. Neither would be regarded as world beaters but both can play in various positions and never fail to give their all to the team, and as a result notch up 40 games a season each on a consistent basis. Osman is like that.
As for his captaincy, who are we to judge? Fans get to see 90 minutes of action each week and presumably judge merit on whether the captain shouts a lot and puts in blood and thunder tackles. I dare say the role is a lot more involved than that and involve much unseen work.
If Moyes thinks he's captaincy material then that's good enough for me.