How does the "Bean Protection Rule" affect Bohnanza?
You lose tension without this rule, since players can effectively just "burn" undesirable beans - both those in hand and those that are flipped in the second phase of each turn. 

My suspicion is that players would trade less and plant more - though without necessarily scoring more points - without the rule. 

The 3rd bean field could be more valuable without the rule, since you are effectively doubling your useful fields (in the case where most players, most of the time, are using their second field for trash). 

The impact on the game length - at least, in terms of how many turns there are - is tied to points scored. More points scored [earlier] --> fewer cards in the deck for each shuffle --> shorter game.