Whether you can get away with building 20% depends on what kind of software you're selling and who is buying it. In the enterprise software world your customers are going to have a check-list of features, some of which are useless or redundant, but they happen to have been implemented by your competitors. If you don't implement those features, you may not even get your foot in the door.

There are lots of other areas where you would be killed if you tried to sell just the 20% of core features. For example, a "20%" smart phone probably couldn't compete against a fully-featured iPhone or Android phone.