Dear Jayaram!
It is a good question.
Infact, the first proviso to Section 112 states that 'Provided that where the tax payable in respect of any income arising from the transfer of a long term capital asset , being listed securites or units or zero coupon bonds, exceeds ten percent of the amount of capital gains before giving effect to the provisions of Second proviso to Section 48 (i.e before applying the indexation of the cost/improvement), then, such excess shall be ignored for the purpose pf computing the tax payable by the assessee'.
Thus, the computation of LTCG can be done at 10% or 20% with indexation, but with one more restrictive condition as in above.
Though this amounts to a little bit of calculations, it is still welcome since it moderates the total tax liability.
Good Bye! Cheers!!