Yusuf was never an ideal specialist batsman. He shouldn't have been played as one. He's better off as a stock off-spinner who can bat lower down. He could have been used as a full-fledged all-rounder in the mould of Gayle, Pollard Afridi, Razzaq, Wright, Collingwood, Styris or Matthews. These players are doing well for their teams, but in this one, Dhoni prefers part-time rubbish over Yusuf, and hardly gives him an innings, so his performance has suffered. He's a casualty of poor planning and unhelpful leadership, more than a very mediocre player.
He's at best an attacking number seven, and possibly the best in a long time, but that means he practically never gets to bat long enough, except when the team is in deep trouble- and he's often rescured them, nearly winning a match each time. His value as a bowler is critical- he relieves the batsmen of bowling duty, and is comparable to some of the more regular bowlers (particularly Jadeja, Praveen, Munaf, Nehra and even Harbhajan- these bowl utter tripe almost all the time and still finish ten overs) but was never given the chance. He has had more bowling innings than batting, so he could have played as a regular bowler, but Dhoni's captaincy seems to give him no room at all. Possibly India's best and most relevant all-rounder, but it seems to take a lot more to please Dhoni.
He's not utter rubbish as some of us make him out to be during this World Cup. He's not of the same class as a Sehwag, Gambhir or Kohli, nor as good a bowler as Zaheer or Ashwin, but as someone who's got the goods to do a dual duty, he's the best you can get. With Tendulkar and Zaheer not likely to carry on after the World Cup, he's surely a lot more relevant than recent dismissals attempting sixes suggests.