I look at it like it was tough circumstances. The Raiders had a lousy O-line and just lousy offense in general in 2014 (holmes last year mostly starting).
They replaced james jones with crabtree on a 1yr prove-it deal, and had the 4th overall pick. The top 2 were QBs so you basically get one of the best non-qbs in the entire draft. Would his numbers have improved if they hadn't selected cooper or brought in crabtree? Probably - but they did and they stayed healthy so his snaps went massively down, and he clearly isn't suited for the slot.
Assuming he played well on teams that shows to me that they valued his depth. He's still relatively young, and was a UDFA so its a hard road to even making a roster in that situation.
Would I like to see more production out of our projected #2/3? Yes. Is he a buy-low possibly high upside kind of guy? Sure.