It only slows down the game and creates less offense.
Home teams become so conservative--like death fearing an away goal.
I'd rather see a shootout, and I hate shootouts.
Can anyone give a good reason for this rule?
Well most people are not too fond of Kicks from the Mark. They would rather them settle it actually playing. I think that's the biggest reason for it.
FIFA did a way with Golden Goals, again teams played very conservative not wanting to give up a goal. Could go straight to an extra 30, but then you are playing a extra 30 on someone's home field. Probably fairer to take into account Away Goals first.
On the other hand it does sometimes really open up the 2nd leg. One team usually has to chase the game.