Rich online apps can learn from old IBM 3270
Convenience, low or no cost bundling and the thrill of "the new" draw users and subscribers into clouds and to other varieties of online applications (Web 2.0, RIA, mobile) and services. By the same token, a shot at recurring subscription revenue, brand/platform stickiness, elimination of piracy, or a non-paying user base primed for targeted marketing is carrying businesses into the online space. It's unstoppable.
Today, all Mac desktops and all popular varieties of professional mobile devices ship with online apps and associated services so deeply wired that they're part of the platform. Over the next couple of years, commercial client software will take on split duties as self-contained applications and "cloud terminals", with appealing functionality set aside for users who subscribe as well as buy.
There's little downside to using online apps as long as you apply sensible standards. Make sure your cloud storage is locally backed up, don't ship anything to the cloud that you wouldn't want shared with the whole world (unless your service guarantees security), guard your online identity, and don't create tacit trust relationships between your LAN and a cloud by, for example, folding your company's shared calendar and contacts into local copies that are synced to Yahoo, Google or MobileMe.