As a designer of software I certainly believe in that. In fact,I have lived that over and over again: Improv learned from 1-2-3, eRoom learned from Lotus Notes, etc. And similarly, BlogBridge has begged, borrowed and stolen from all sorts of other applications, including NetFlix, Del.icio.us, Wiki's, numerous email clients and on and on. And not by accident but by conciously studying what others have done.
That said, there are certain design choices which you can't straddle: they are fundamental - you have to pick. To bring it home with an example, many people far prefer web based aggregators (like Bloglines) over client based aggregators (like BlogBridge.) Each brings their advantages and will appeal to a certain subset of users, and this is important: they will be a turn-off for another subset of users.
As a designer you have to make such choices while knowing that you will alienate some of the users you would hope to get, but counting on the fact that there is enough diversity in taste and work style to allow more than one model succeed.