aussie
Hall of Fame Member
Its isn't logical for you.Basically if you are picking a keeper/batsman, you evaluate him based mainly on the matches he actually played as a keeper batsman. To pick Sanga to keep wicket and bat 5 based on his batting 3 isn't logical.
IMO its very logical to do so because he was very adaptable player like many others such players in test history. We saw him pretty much throughout his ODI & T20 career (which coincided with his test batting peak after 2008), bat @ 4/5 & maintain his world-class batting standards.