The key benefit is that you can shift some of the cooling requirements to the Delta fans, which are designed to run 200+CFM for ever, rather than GPU fans that will eventually fail (albeit after a lot of abuse). In practice this means you can throttle the GPU fans right back, moving heat away from the cards even more. This also means that if a GPU fan does fail, you have redundancy.
On the downside, they're louder than open frames, and you'll still be limited by the thermal capacity of your in-and-out fans, so you will get a denser setup, but not a bigger one (assuming you are currently maxed out).