Timvp has assembled an impressive case against Forbes but I'll push back a bit.
Most teams, good or bad, start a guard who is weak on D. It's kind of like having a starting pitcher who can't hit -- when comparing such pitchers, you don't talk about which pitcher hits worse (who really cares) you talk about who is the better pitcher. That's what they're paid to do and that's their value.
Forbes' value will be determined by his offensive performance -- especially on a team as desperate for three-point shooting as the Spurs. Take a look at Game 7 of the Denver series and then straight-face the claim that he hurts this team.
And if you want to evaluate weak defensive guards within a team defense (like the Spurs'), I would suggest you start with looking at how often that guard is out of position. Forbes holds up fairly well under that standard. For example, in both of the clips in the article, Forbes was between his man an the basket. Small praise but at least better than Lonnie Walker who often wanders around close to half court like he's playing a one-man zone against the Jumbotron. (And with Walker's offensive potential, he shouldn't be evaluated only on his defense either.) In short, staying in front of your man doesn't break down a team defense nearly as badly as completely losing your man does.
As far as attempting to extrapolate Forbes' defensive play by looking at team defense statistics when he's in, I think that's a reach. I'm not a fan of +/- analysis and such stats seem to be venturing dangerously into that territory. Believe your eyes -- Forbes seems to always hit a big three when the team most needs it, when the team is going bad. Those are statement shots that can affect the game much more than the same shot in a less critical situation.
Pop has taken a lot of heat here lately during this losing streak, some of it deserved. But when it comes to evaluating defense, I'll defer to him every day of the week. Pop absolutely will not play anyone who misses defensive assignments and yet he starts Forbes.
So is Forbes a defensive liability? Absolutely. Does he hurt the team as a starter? I think not.