Some guys like to take the chair lift to the top of the mountain, and ski groomed runs over and over.
Other guys like to climb up the mountain themselves, and ski down new terrain they've never seen.
Both could be great skiers, but neither does the same thing as the other.
The advantage I find with the hand call is, as stated above, inflection and variety...
with the e-caller you call fifteen to thirty seconds of the same sound for an hour. With the hand calls you call for an hour without ever making the exact same sound more than once.
Also no two mouths are the same, so the sounds from a hand call will sound different with different players playing. Not so with e-callers... if a guy was here recently with the same tape you have, the critters have "got your number" they've heard it before.
Even volume is not that huge of a consideration, I've got hand calls that are so loud (if played at full blast) that can make my ears ring if played in the house, so we can all be too loud.
Jeff