I found : http://en.wikipedia.org/wiki/Voter_Verified_Paper_Audit_Trai...
for a country of our size with 120 crore people, it already takes multiple days of voting in phases, due to security and administrative difficulties
if we go back to paper ballots, counting will take many days, error prone, we would have large number of invalid votes
booth capture used to be norm in UP bihar in 1980's early 1990's