This is a great question. The Keller funnel has become a very common part of my practice for several reasons.
One of the thoughts behind capsular contracture is that the implant has some small amount of bacteria on the implant that causes the body to have a reaction and form the capsular scar tissue.
I do believe that the funnel decreases the chance that the implant will have any contaminant on it. The funnel keeps the implant from ever touching the skin. I have never seen a true study that compares the risk of capsular contracture in patients that have had a breast augmentation with a Keller Funnel and those that have not had it with the funnel.
The other benefits of the Keller funnel is that you can place the implant through a much smaller incision than can be placed with the traditional method. The funnel also avoids unnecessary trauma to the implant or the skin. This is especially important when used for larger implants or through a trans-axillary augmentation.
For these reasons I typically use the Keller funnel for both primary augmentations and revision breast surgery.