In boxing we are taught the importance of a good, tight guard. I see alot of self defense experts not really bother with it. For example, they will use the fence to try and disarm the situation and if it breaks down unleash a world of hurt very quickly. In karate there is no guard, instead focusing on jamming shots and getting in close. Having your guard up I guess means your being defensive, and therefore on the backfoot. While in a ring you can sit back, take shots on the gloves and counter punch those things wont work in a street fight so you either need to APPEAR (not be) passive, as in using the fence, or being aggressive so that you can use momentum to carry the advantage of the pre-emptive strike if possible.
I find the guard in mma and the especially that used by early MMA fighters and BJJ specialists very interesting. They have their arms extended more, like bareknuckle boxers (look at gypsy fights) so that they can intercept punches on their way in and clinch up. that said, there must be a use for it? right?