MSDN: Software as a Service -- in which, once again, the technologist's dream of variable pricing is promised:
Because it's simpler that way.
"Most of the features in these applications have been added in order to satisfy some segment of the user community. But what if you could set the application up so that it only included the features that you used? And not only would you pay just for the features you used, but you could also further reduce the expense by only paying as often as you used a particular feature. Perhaps you only use mail merge once a year to send out your infamous Christmas letter; why pay for it the other 364 days of the year?"
Because it's simpler that way.