1. Ath and Spd are both important in the defensive equation for guards. They are close to equal when pressing but generally ath is more important. I think, and I don't know if this is widely accepted as fact, that ATH is used more in defense in 2 point shots and SPD is used more when covering 3 point shots.
2. Speed is more important on offense for players with higher PER scores. ATH is more important for players with high LP scores.
3.In D3, I think you could get away with a 30-60-30 (ATH, SPD, DEF) if he has a really high PERIMETER score (70+) if you are trying to make the NT. IF you are trying to win the NT, the speed and perimeter score should be in the 90's like Yanks guy.
4. I agree with Blackdog, those types of guys are best used off the bench. It's hard to tell how many points bad defenders give up but he will get exploited in a man to man defense by a competent coach.