As a three sport athlete Danny tends to go with the "best athlete".
Is that why he picked Glen Davis?

I don't know exactly what Danny's draft philosophy is, but it seems to me that if you're drafting late, you should focus on guys who may not be all-around great players, but who have one or two defining abilities that are likely to translate to the NBA level.
In other words, you look for guys who are likely to be good role players. You look at guys that get drafted late who tend to stick, and it seems to be players of that sort. DeJuan Blair -- undersized, but a heck of a rebounder. Matt Bonner -- he's big and he shoots the lights out. Gary Neal -- shoots the lights out. Landry Fields -- boards his position really well and shoots well, plays within himself. Those are just the examples I can think of from the last couple of drafts (except for Bonner).
For every DeAndre Jordan, Carlos Boozer, Monta Ellis, or Manu Ginobili (good to great all-around players) who get selected later in the draft, there are at least a handful of capable, quality role players who are available late.
Moral of the story being, I think personally I'd go with the guy who has shown a great knack for doing one thing really well (in this case scoring) over a guy who might be a real good overall player down the road if he can develop his body, but who might also be a total dud who just doesn't do any particular thing at an NBA level.
But I'm not an NBA GM. We'll see a year or two down the road which of Marshon and JJJ is the better NBA player. At the moment, though, you have to give the nod to Marshon if only because he's actually in an NBA rotation.