It was a novelty on N64, but I probably only turned it on in Multiplayer 3-4 times out of thousands of matches over the years...
The increased hitboxes for heads would have a massive - possibly game-breaking - effect on how GE:Source multiplayer works... if it were ever implemented, sure it would be fun to play as a novelty now and then... but I'd hate to log on to the server browser one night and see 6 populated servers, 4 of which have DK MODE ONLY!!!! running...
Already have issues with servers messing with gravity and other settings at the moment - which is also fun, but it gets to be annoying when there's 4 populated servers and 3 are anti-grave or running mods which totally change the balanced, classic GoldenEye gameplay many of us are looking for when we log in to the mod.
Would it be cool to see, simply because the original game had it? Sure. But I'd be careful what you wished for with this one...