For a document that the visitor cannot afford to lose, or for anything in which privacy, security or personal control are issues, a Web-based document is obviously a mistake. Why do you think the big players in cloud-based and Web-based storage and applications are having such a hard time getting people to adopt them?
The answer is obvious -- the drawbacks greatly outweigh the advantages. The risk of losing sensitive content or having it be compromised or stolen is too great.
Even games, a less serious endeavor, suffer when they adopt a Web-based approach. Look at the brouhaha that followed from making the most recent SimCity version work online-only:
http://www.gamesindustry.biz/articles/2013-03-15-ea-defends-...
A quote:
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"So, could we have built a subset offline mode? Yes," Bradshaw admitted. "But we rejected that idea because it didn't fit with our vision...The SimCity we delivered captures the magic of its heritage but catches up with ever-improving technology."
A number of upset fans in the comments section to Bradshaw's update were not assuaged by the explanation. As one user going by the handle klymen wrote, "With all due respect, to write an article about why the new SimCity has to be always online and not to mention DRM or the anti-piracy measure even once, is dishonest and downright disrespectful. It seems like that you don't think very highly of your audience. Yes, the DRM issue is a sensitive topic, but to avoid it, not to mention it as one of the reason why SimCity is always online, shows absolute disconnect and contempt for your fans. We're not stupid."
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The above is just about a game, not a business transaction or potentially sensitive communication.
As to data loss, consider incidents like this:
Title: Amazon's Cloud Crash Disaster Permanently Destroyed Many Customers' Data
Link: http://articles.businessinsider.com/2011-04-28/tech/29958976...
Quote: "In addition to taking down the sites of dozens of high-profile companies for hours (and, in some cases, days), Amazon's huge EC2 cloud services crash permanently destroyed some data.
The data loss was apparently small relative to the total data stored, but anyone who runs a web site can immediately understand how terrifying a prospect any data loss is."
In conclusion, and to second the SimCity gamer's quote above, we're not stupid.