In a cover 2 situation, if you've got 2 recievers on the same side of the ball, that safety will most likely cheat to that extra reciever, in a base package, with a linebacker providing underneath coverage. Depending on how the recievers leave the line, and what kind of moves he makes, the safety will most likely leave that corner on an island. In most cases, the safety is reading the QB, trying to figure out what he's going to do. If you've got a good pass rush, then the QB will be forced to make the first move, giving the safety the advantage. If he's got time in the pocket, the safety has to make the first move, and looses the advantage, unless you've got a true shutdown corner...... my player is better than yours. Then the safety can forget about the covered reciever, and focus on the other. Release a tight end into the pattern(shockey, Dave Thomas, Witten, etc...) and the safeties job get's a lot tougher.
Bringing the safety down for run support(as opposed to cover 2) gives you a bit of that 3-4 unpredictability.... is the safety coming, or the SLB, WLB kind of thing. Can that safety fall back into coverage fast enough??
Personally, I prefer the 3-4......... always have. I think you have more options, more chances to cause confusion, which puts more pressure on the QB.....