That's a pretty lame rule if I may say so myself. Kinda of violates the whole spirit of the game. You have to use real words that you know how to spell. Requiring a definition, especially one that matches some chosen dictionary, opens up a lot of weird challenge problems. Definitions are much more fluid than spelling (although spellings can change over time as well). Spelling and whether or not a word is "real" tend to be much more cut and dry.
I would say the spirit of the game is a competition based on vocabulary. Surely memorising character sequences with no understanding is completely against that ideal?
I agree however, that requiring an exact definition would be a bad way to do it. The point is to understand, not to memorise, so they should only need to give an approximate definition. For casual games, I think agreement of the players is fine, and for competitive play, you can have a judge to decide if their definition is close enough.
Not a big worry. My guess is for unusual words, meaning variance is pretty low because their use (what fixes the meaning) is low too. If you find out the meaning the player attaches to the word doesn't match the dictionary, you can be pretty much certain he is trying to bullshit it (assuming the rule is in effect, it wouldn't be a problem under the usual rules).
I think the rule just adds some more challenge to the game.
