That said, the Indians surely need a fifth bowler. Their bowling attack, with only four bowlers, is not strong enough to take 20 wickets consistently anywhere. Moreover, the bowlers get overworked once too often, and a few get injured or carry injuries throughout the series. Irfan struggling for form is disappointing, but someone has to do the job. If the Indians choose not to play five bowlers, they can have a batsman to play as a fifth bowler.
Of the top six, only Sehwag really enjoys bowling, but that's about it, and we don't know if any will last more than six overs an innings. A 'joker' option like Rajat Bhatia or Abhishek Nayar is also worth a try as both are very capable batsmen (good enough for number six in this order) and can contribute more than a few overs- enough to give the main bowlers a rest. The idea is not so much to outperform the top four bowlers, but to support them and add to the wickets tally.
When you have a batting lineup so strong (and also a bowling lineup so weak), you can take a chance with an all-rounder or a fifth bowler, but you need to have Irfan back in form. Moreover, Harbhajan and Zaheer are quite good with the bat, on their day, and need to put their batting skills to some use. Bowlers like Amit Mishra, Piyush Chawla and Praveen Kumar are still young and raw, and can score a few runs in addition to taking those wickets. Rather than be so cautious that they can't strike, the Indian team needs to be stretched a little, and these prospects need to be given a few chances.