Terran being defenseless at initial stage? I dont think so. The ability to barricade their ramp is sufficient time to wait for marines to come out. The ability to repair their structures to buy time is another huge advantage to terrans. 2 marines shooting behind a wall will stop a bunch of zerglings and zealots from initial attack.

Its worth while to invest the 1st scan into your opponents base to find out what their plan exactly is. constantly watching them is good because you need to understand that the mule is added advantage in economy where they are still going at normal mining rate (protoss n zerg). Knowing their move and countering is deadlier. For example an early fast expansion scouted, send your marines and marauders in and head for it. Let it construct and then proceed to take it out. You have set them back 300-400minerals and that's worth 1 mule as well.

Engineering bay is something people often neglect, but its a +1 for marines and if you are massing marines and marauders, the value of +1atk upgrade increases significantly. Its also good to place a turret or two around your mineral line to prevent such harass by banshees or dark templar warp ins. And if you have used scan on opponents aggressively you would have spotted a banshee or mutalisk or void rays build early in which 2 turrets + couple of marines will be sufficient for early air harass.