Com Sweep doesn't do shit to stop DT rushes.
They're only visible for a few seconds, and they can just run away and come back. Com Sweeps take a ton of energy, most of which should spent on mules as Terran early on.
The orbital command upgrade takes plenty of time to research - time you're not building extra SCVs. Protoss on the other hand get to Chrono Boost their Probes. Early Terran economy relies on the mule drop.
If your CC is hit by DTs and you lift it off to save it, you then can't use the Com Sweep to scan for and finish off the DTs. Missile turrets go down so fast, a single one isn't much of a deterrent, either.
An early Terran player basically has to choose between killing his economy by throwing in the Engineering Bay and ponying up for several missile turrets (you need at least 1 by your ramp and at least 2 by your CC if you don't want to get destroyed), the armor upgrade, and probably the range upgrade, or risking it and running naked against stealth for a while.
As Protoss, if you have 3 or 4 DTs reasonably early, there's nothing Terran can do.
If you can warp in (or walk in) unnoticed (players CAN see the stealthed units moving, at least on the highest graphic settings), simply sit by the CC and wait for them to mule drop. Then move in. If they're keeping spare energy for a scan, move 1 DT in, then run when the scan goes off. Then move them all in afterward.
If you can't warp / walk in unnoticed, just do a few hit and runs on the wall. Each scan used is a significant setback to the Terran economy (because it's not spent on a mule). If he puts up turrets behind his wall, that's an even more significant hit.
They should make the Com Sweep take 40 energy instead of 50.
(And they should make the supply drop thing actually useful somehow.)
As Terran, I usually am liberal with the missile turrets if I'm fighting Protoss.
Typically, though, that investment will screw you bad if you're going against Terran, and worse if you're against Zerg. Of course, you don't need those turrets as much against Zerg or Terran. But I don't play 1v1, damn it!