so far, i've never seen any1 MMM with siege tanks, they are way to slow, and most of the time they will do damage to your own units if enemy do melee, the purpose of siege tank should be for terran to fortify a certain area, it takes quite alot of resource to build a tank, and it need to be researched, plus it takes time to siege/unsiege, so i think it would be fair to have more benefits in siege mode compare to unsiege. I think increase the time to siege/unsiege ie 5 secs, would be better than reduce the damage by that amount.

as for zerg, ya i think they are pretty weak especially about defending, i think blizzard knew about it and that is why there are some bugs after you destroyed a zerg building. but that doesnt help much, in game ie 3v3, protoss and terran can easily fortify and seal their entrance, but zerg cant do anything about it unless they tech to lair quick and use overlords to create creep(queen also work, but if the distance is far, overlord better), then make buildings or spires the entrance, that needs alot of resource and speed. And another weakness of zerg is their overlords, unlike terran/protoss their supplies based on building, and those buildings have quite alot of hp, so you might need some time to destroy those building, but overlords are very easy to kill, and usually they are all pack together, and if enemy found that, most likely you will lost all of them, and you are basically in very very big problem. ya overlord got advantage too, they can move, but the speed is way too slow unless researched speed. and 1 feature i really hope they can add in, which is allow us to have auto-cast create larvae, so we dont look at the base every minute. the tip says "you can create units without looking at your base", that is partially wrong for zerg that uses queen.