I think that there are some things that should be considered before saying that Strikeforce couldn't benefit from this.
1. Could lead to the possibility of PPV, which has been mentioned.
2. Could lead to a situation where the Bellator tournament setup leads to options for lower level SF fighters to get more coverage by adding them into the tourney. Wouldn't it have been better for Strikeforce for Lashley to be exposed on a lesser known show then for them to throw so much money into marketing him before knowing exactly what they had.
3. Better TV coverage- Bellator doesn't reasonably well on FSN and Strikeforce does reasonably well on Showtime but together they could push for a better time slot and possibly a better station, maybe network TV.
4. There just aren't many really good options for some of the SF champions within their own organization. I mean who really wants to see Melendez fight the punk again. Melendez vs ALvarez would sell like crazy for those of us who actually know anything about MMA. Who is left at 170 in SF that even poses a small threat to Diaz so why not have him fight Askern who would be the best wrestler that Diaz has ever fought and poses a significant threat.
Those are just a few of the potential benefits of co-promotion, not to mention the possibility of absorbing some fighters and the increase in talent pool that could follow. If these guys ever want to compete with the UFC they need to do something like this.