It looks like the advantages of changing right now are "none at all" inasmuch as the project has only just been added to the kernel, has very little documentation, and is self-described as being "still in its dipers[sic] atm".
In the long run, a userspace networking bonding driver could have some of the same benefits that FUSE (the userspace filesystem interface) brings to the world of filesystems -- primarily that it's much easier to develop and experiment with different policies, protocol implementations, and so forth. By simplifying the in-kernel code and pushing the complexity into userspace, you can also end up with a solution that is more robust in the event of failures and that allows for more agile responses to bugs and feature requests and so forth.
This presentation (warning:PDF) describes the motivation and goals of the project. Primarily, they're looking to replace the legacy bonding code which is bloated and complicated with something that is smaller, easier to maintain, and more performant.