Why heal your allies when the team is flying through the air silenced by your scream, and being feasted 1 by 1? Cho'gath needs to be able to aggro the damage to himself before others, so heal wouldn't be as useful as per say, teleport which brings you to teamfights, ganks, and farming lanes.

The flash is very helpful for escaping. Also, the heal is more for your allies than you. And the extra experience and mana from the utility masteries gives a huge advantage in leveling and having the ability to stay in the field.

should fill defense mastery to be an actual tank. flash 15 seconds earlier not that important. you don't need heal if you have force of nature and heal from every minion kill.