This[1] RAND paper indicates perennial polyculture farming could increase yields for poor, third-world, subsistence farmers. Which is a very good thing.
But it seems unlikely to approach the yields of annual monoculture farming. (One quote: "The biggest difference, however, comes from considering perennial cereals. Most of the cereals that people eat (such as wheat, rice, oats, and corn/maize) are grown in annual plantings and often in monocultures. Since cereals account for at least half of dietary energy world-wide, converting that production to perennial polycultures with mixed intercropping would be a significant change in worldwide agriculture.") That's a bad thing; I'd like to see more use of these techniques, for all of the primary benefits mentioned in the paper. (The yield thing for subsistence farmers is a secondary benefit.)
But it seems unlikely to approach the yields of annual monoculture farming. (One quote: "The biggest difference, however, comes from considering perennial cereals. Most of the cereals that people eat (such as wheat, rice, oats, and corn/maize) are grown in annual plantings and often in monocultures. Since cereals account for at least half of dietary energy world-wide, converting that production to perennial polycultures with mixed intercropping would be a significant change in worldwide agriculture.") That's a bad thing; I'd like to see more use of these techniques, for all of the primary benefits mentioned in the paper. (The yield thing for subsistence farmers is a secondary benefit.)
[1] And it would help if I added the link: http://www.rand.org/content/dam/rand/pubs/occasional_papers/... [PDF]