Ionut Alex Chitu of the Google Operating System blog has a good point about web-based software services like Gmail and Google Docs: You often have no choice about whether to use an upgraded version of the software or not. For example, Gmail recently went through a major upgrade that many people were unhappy with for various reasons, but they have no option to keep using the previous version. Contrast this with desktop software — I’m still using MS Office 2002 as I had no real need to upgrade to Office XP or Office 2007.

On the other hand, upgrades of web-based software are automatically compatible with previous versions. If Google Docs updates, all your documents will surely be accessible in the new version. Thus web-based software cannot use the strategy of pushing out new versions with incompatible file formats, to force people to purchase an upgrade.