I'm half and half on it. If they tweak those numbers to something reasonable (lower % taken off starting, but then gradually becomes more and more, and could possibly cap at 25%? This way you still get decent gear to try and farm. 25% may be too high, again, it could be lower. Just some figures for comprehension! :) )
The percentage ratio should depend on what is expected of the player ie: difficulty. On the beginner maps the loot won't be the best, and these are the places where those unfamiliar with DD will test around so they shouldn't be punished that hard, and I think the scale should progressively get harder as you ascend through the levels.
Once you start to compete in high tier, high reward and high difficulty mission segments then the punishment should be ruthless for extra towers if the drops you're going to be getting are some of the best in the game.
Done this way it lets the new players and more casual players experiment with builds and their own way of doing things, while easing into a more difficult style of play. Along side that the hardcore and more dedicated players get to think out strategies for harder levels therefore giving them maximum loot potential and still having a difficulty curve too it.
Now if done correctly, the casual and more veteran/hardcore players can meet in the middle and will eventually be able to play with each other without a problem because the casual players themselves have been slowly moved into a more thoughtful style of play, giving headroom for strategies on both sides.
and even if people want to just play the harder maps for fun, then nothing is stopping them from using more towers than the game wants them too, it just means less reward for them, and that's not that bad when you think about it.