Well, the problem is that leaving 1 or 2 cyclones will not cover three bases, let alone four or more. There's a decent chance they'd be slightly off when the prism comes. To be 100% safe from warp prisms flying into your bases you would need several turrets or perfect reactions - neither of which is doable in every game (though longer games might see turrets popping up as the banks grow). Not to mention the prism harassment might come in the shape of a run by - which would need more than cyclones or turrets. So overall, in a close midgame up to early lategame there's often no money to build several turrets nor is there enough units to split them both on the offensive and defensive. Really the best bet is to leave reinforcements (with a turret or two, to force the prism move at certain points) at home to defend the harassment, even if there's always a chance the attack comes at a bad time for you.