I've never competed in full contact officially, but I have done it with some friends. While walking up in pads and all may not get the perfect feel for taking hits, it's far superior to point sparring in which you never really feel it. The pluses of full contact are that you learn to hit full force, and therefore will hit full force in a real fight. Someone who fights point sparring is constantly building the habit of pulling his punches, and, as a result of habit, may actually end up doing that in a fight even unintentionally. Yes, he may THINK that he could hit full force, but if he's trained to tap the opponent, he'll need to be consciously reminding himself to go all the way through on his techniques, as opposed to someone who has trained Full Contact who will do it naturally.
And the other plus to Full Contact is that you learn to take hits. As someone already said, if your covered in pads it isn't QUITE the same. However, you're still learning to deal with the impact, and depending on your school (boxing schools for example, do not use many pads) you're learning to deal with the pain of getting hit.It really toughens you up. Point sparring, you never get used to the feel of getting hit.