One trick pony ...and not an essential one
Pinterest does one set of tasks and that's it. It is not comparable with an enterprise application that does a multitude of things. Further, Pinterest is not a business critical app for anyone...except for the makers of Pinterest. That means if Pinterest doesn't work right users do not lose business from it. Comparing Pinterest to enterprise (web) apps is comparing apples to oranges.
Many enterprise customers even dislike quarterly releases and often opt not to take them if they have control over it. They rather have annual releases that can be installed when there will be less impact on business. Releasing early and often is nothing more than a sales tool, it allows sales reps to show new features to potential customers quicker and it allows the business to add features faster to potentially close a deal. Often times these deal enablers end up being just a Potemkin's village. The release often craze has some benefits, but it is a major drawback on application quality. Proper design and testing are out, let's have (paying) customers test it. So what if they get dissatisfied? Since everyone is pushing out mediocre apps at rapid pace the customers are getting used to increasingly crappy software. Apparently, this is where folks want to go.