Beno is by no means the biggest problem on defense, but he is a problem. And the problem is symptomatic of the team as a whole. He's competent at the position but he doesn't do anything to make other guards uncomfortable. And what he gives the team on offense, most nights, doesn't counteract his limitations. His mid-range game and craftiness around the basket are assets to the team, and would be on any team, but if you're not a first or second option, your value in a rotation goes down dramatically when you're not playing a role defensively.
Not everyone out there needs to be a defensive stopper, but you need to do something well. Doug Christie wasn't the best man-to-man defender, for example, but he made it difficult for other teams to run their plays because he played the proper angle on the passing lanes to make it difficult for his match up to get the ball. Even if you can just take away one option for the offense, like Donte does with his length, you're forcing the other team to alter their strategy and that's going to result, more often than not, in more favorable opponent shooting percentages.
Tyreke is a good on-the-ball defender because he's got quick hands and good timing going for steals. He sometimes over commits though and let's his man get by him as a result. His long arms also help him to play the passing lanes. But then he doesn't have the best lateral quickness either so he can be beat by faster guards. So the second guard should be able to match up with smaller, quicker players or have enough length and awareness to play the passing lanes. Beno does neither well. Thornton is slightly bigger and a little better in the passing lanes. Either one of them would be acceptable off the bench, but our perimeter defense will always be a weakness with that rotation. We badly need another guard who's an above-average defender. Hopefully with some decent size as well. Garcia has the length to play that role, but he's being played at SF right now instead which, sadly, makes our lineup even smaller.