It's not actually a preview of the series. It's a bonus comic that includes two original short stories that won't run in the regular series and won't be reprinted until the trade. So if you plan on collecting all the Fraggle Rock comics that come out as they come out, you definitely want to pick up the Free Comic Book Day issue! (Plus, it also includes a new Mouse Guard story, written and drawn by David Petersen, who draws those Muppet comic covers you all know and love!)
As for releasing them so close together, that was just a fluke of scheduling. Free Comic Book Day falls very early in May this year. I realize that may mean making two trips to your local comic shop this week, but isn't it worth it for free comics? (Especially Fraggle Rock ones!)
Tim Beedle
Editor, Fraggle Rock