We always play a scrabble rule where you can challenge someone to give a definition of the word they "claim" is real ... and if the definition is not in the chosen dictionary, then they are not allowed it ... even if it's a real word.
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.
I think the rule just adds some more challenge to the game.