What constitutes a rolling chassis motorcycle really depends on where you're buying it from. Some rollers may just be Sportster frames with a front end and some spoked rims, while others include full sheet metal, and can be set up for wide tires, right side swing arms and possibly have front ends that cost as much as some complete rollers.
The real advantage to building a chopper from a rolling chassis is that you're not only not limited to what you can do to the roller, you are, in most cases, not limited to what you start with. Most companies allow you to specify the rake, stretch, what motor mounts you want as well as the tire width and style of gas tank you would like.
Other big name companies like Big Dog offer rollers of their production bikes with less available options for the frame while still giving you creative freedom on a base that you know will work. Redneck Engineering is one of those other companies that will pretty much make any roller you want.
First time chopper builders may want to play it safe with a roller rather than risk buying sheet metal or wheels that won't work with the frame that they have. And they will also know ahead of time exactly how the tank looks with the frame and front end. Takes the guess work out of making sure the lines of your chopper end up being what you envisioned in the first place.
So for the custom chopper builder who doesn't want to have to select every individual part but also doesn't want to have complete bike kits sent to him the rolling chassis gives the option of having your bike only as pre-designed as you'd like it to be.
