I would strongly advise against moving your own domain name to anything other than another domain that you own. You will lose a small (minimal) amount of link equity from the redirect, but that is not the big reason. Several of the primary reasons I would encourage you to weigh this decision carefully include:

• I want to control and own all of my content. An example of how this could go very badly would be if you violate the Weebly terms of service (TOS). In several extreme cases this has led to a whole site begin deleted with other similar services (i.e. due to copyright infringement, etc.). Obviously, this is a far-fetched example, but I am a firm believer in self-hosting and owning/controlling all of my own high-quality content. Another more feasible example is if Weebly went out of business or was acquired and you didn't have time or know to migrate out.
• Less flexibility in hosting options and plans. Recently, a brand I work with saw traffic grow 30x overnight due to positive press coverage. We simply "dialed up" our cloud hosting plan and everything went smoothly - no downtime. This was with simultaneous coverage on the front page of Digg, Gizmodo, Mashable, NY Times, and a ton of other sites. Had we had a normal hosting plan with Weebly or other website builders our site would have crashed.
• Zero or minimal options for developers. Root access and FTP access is usually blocked with website builder services.
• Branding considerations. Do your customers already know you by your domain name? Does it make you appear like a smaller player since you are on a .Weebly domain? There are a multitude of branding considerations as well.

I would look long and hard at why I am thinking about moving (i.e. I like the builder) and see if there aren't other good options that would allow me to stay on my own domain rather than a .weebly domain. You lose a small amount (minimal) of SEO benefit, but more importantly lose control and options.