Well, stats really don't tell you the whole story here. Crabbe is ready to breakout from Lillard/McCollum. Casspi has been in the league for 7 years, and we already know what his role is. Hell, the reason why he left in the first place was because of the refusal to accept his role. The rest of his career after Sacramento really gave him a harsh reality check.
You can't just put up numbers without the context. It's really unfair to Omri too.
I think Omri can be a starting SF, but he'd have a lot of trouble on defense. He has the length to guard SFs, but he doesn't have the strength. If you ask him to bulk up, I think he'll lose a significant amount of quickness, which will hamper his ability to finish at the rim. Omri is a guy who really excels in the 2nd unit, because he's can play more freely. I don't know that he'd be able to play as loosely as he does with starters.
If Omri is our starting SF, we better have an elite PG or SG who can score. If we take Gay off our team, we arguably have no real #2 and #3 scoring option. DC is a fringe #3 scorer....
Well since it's been about 5 years since he left the first time I'm not sure how relevant it really is...
What I do find atleast somewhat relevant is that Casspi played significantly better over the last 2 years as a starter:
2014-15 (19 games starting)- 14.3 pts (52.9 FG%, 50% from 3), 4.5 reb and 2.2 assists on 1.9 TO's.
2015-16 (21 games starting)- 14.0 pts (50.5 FG%, 48% from 3), 7.0 reb and 1.7 assists on 1.8 TO's.
So when he was called to have a bigger role he did it and his efficiency actually went up significantly, Another thing is that the most effective 2 man combination for our team this year was Casspi-Cousins.
I'm not sure at all that he is worse defensively than Crabbe (there aren't numbers suggesting that and the eye test for me is that Crabbe is overrated defensively- he has potential but he isn't there yet).
Now you can say that Crabbe has higher upside and that he is waiting to get away of CJ's shadow, on the other hand he only had one good season in his short career (his season high prior was 3.3 points) and in that season he didn't produce as much as Casspi.
Another example is Bazemore- a guy only a year younger than Casspi, who people look at as someone that can bring immediate value on offense- now to be clear: I think Bazemore is the better player, but production wise Casspi is very similar:
Casspi: 11.8 pts (48.1 FG%, 40.9 3pt%), 5.9 reb, 1.4 ast and 1.4 TO's in 27.2 minutes. PER 13.96, RPM +0.96 (16th amongst SF's)
Bazemore: 11.6 pts (44.1 FG%, 35.7 3pt%), 5.1 reb, 2.3 ast and 1.8 TO's in 27.8 minutes. PER 13.42, RPM -1.34 (39th amongst SF's)
So again with almost identical minutes (Bazemore played slightly more) Casspi have far better shooting%, 0.2 advantage on points, 0.8 reb, 0.4 less TO's, better PER and far better RPM with Bazemore having a 0.9 advantage on assists.
Now again, Bazemore is the better player here overall but Casspi gave the same production with more efficiency and you are going to tell me one of them is a near-max guy who would be a home-run and the other one can't be considered as a starter?