not only would environement details like that cause more gaming and battle disruption it would ruin the idea of valve making the game available for low grade computers, details like that eat alot of CPU/RAM Data to create such effects, such as breaking walls, tree's , etc. thats why games like bf bad company 2 require such high grade computers
valve are doing there best, and good on em to do this, to provide the game for high grade computers, and those really really old computers that can only run the original dota. so everyone can play the game