Look, i've been driving the "Forbes is trash" tour bus, but think that he can at least be passible as a starter playing spot minutes. He isn't a good defender, really can't and doesn't pass- with those flaws known, he can knock down 3's and play off of Aldridge and DDR drawing attention, that's something the Spurs need in the SL.
If Pop does start Forbes then it sounds like he is trying to balance out the SL and bench units. A bench unit of Mills, Beli, Gay*, Bertans, Gasol** is a very good bench all things considered- plenty of shooting, good cutters, passers and a one on one scorer.
*-I expect Gay to start but also play with the bench.
**-Pop may start Poeltl in a Blair role, Gasol's skillset is better suited off the bench anyways.