David Bordwell had a great post on his blog pointing out both the historical precedents for transmedia storytelling and the problems inherent in it. You should definitely read the whole thing, but this gives you a nice taste:
At this point someone usually says that interactive storytelling allows the filmmaker to surrender some control to the viewer, who is empowered to choose her own adventure. This notion is worth a long blog entry in itself, so I’ll simply assert without proof: Storytelling is crucially all about control. It sometimes obliges the viewer to take adventures she could not imagine. Storytelling is artistic tyranny, and not always benevolent.
Another drawback to shifting a story among platforms: art works gain strength by having firm boundaries. A movie’s opening deserves to be treated as a distinct portal, a privileged point of access, a punctual moment at which we can take a breath and plunge into the story world. Likewise, the closing ought to be palpable, even if it’s a diminuendo or an unresolved chord. The special thrill of beginning and ending can be vitiated if we come to see the first shots as just continuations of the webisode, and closing images as something to be stitched to more stuff unfolding online. There’s a reason that pictures have frames.