put it this way. easier all rounder units.

1. gateway + cybernetics core = stalkers, pwnz everything......... pretty much. if you've seen how the pros play, zealots + stalkers (or last time, dragoons) were enough for lasting you through the game.

2. stargate you can bring in those rays, damn powerful also.

3. 1 probe can go around build stuff and go back to harvesting.

4. upgrading the production building doesn't require a pause. for example you can continue spamming zealots while waiting for cybernetics core to build.

hehe. see how your zealots and stalkers handle zerglings and roaches coming at you.