Trees give realism. Deleting them would also end most of GV actions.
Actually, GV vs GV combat took quite a hit when AH3 with its much greater coverage of trees & bushes dropped. For example, it instantly killed off the formerly very popular Tank Town on CraterMa, because it now suddenly was much more a game of Hide & Seek, with an emphasis on "hide".
AH had previously a notably higher percentage of GV on GV kills, and a much higher k/h in the same aspect.