cookie monster:
In chess, a player with a material disadvantage may play for a draw rather than resigning (a draw is better than a loss). With the 50-move rule, it is up to the player with the material advantage to advance the position (a win is better than a draw). It's a good idea in an inferior (unlikely winnable) position to play for a draw. That outcome is more reasonably attainable with a draw rule.
K+B+N vs K endgames (in which the B & N aren't in a trap position where one can't avoid capture) are winnable in less than 50 moves by a skilled player.

If by "arbitrary" you mean 50 moves instead of 46 or 53, I agree. Otherwise, based on the things I've read about the history of the rule, I don't think the rule is arbitrary. I think it was well thought out. It takes into account the standard mates and a margin of error for accomplishing them as well as the moves for pawn advancement/promotion and captures.

There is already a 50-move precedent for an Espionage draw rule (35 for the small board variations). Deviating from that precedent is arbitrary if not capricious.