So you’re using presets on your Prophet, albeit by a great sound designer and musician, how about that? 
A lot of the best musicians of the golden days of synth music used to pay someone else to create patches as raw material for their tracks, so yeah sometimes I do want to outsource that so I can focus on creating music and not gear tinkering.
I don't think I've ever used a preset in its original form, on my P10... they get heavily tweaked along the way, but sometimes do as a jumping off point. My interest in external sound libraries is partially because I like to deconstruct patches others have made, in order to learn the elements that comprise them and introduce someone else's ears into my own sound design process. Other times I just want a wide variety of timbres to select from quickly.
But yeah I think it's very worthwhile to load sounds others have created. We all tend to form our own patterns and preferences when creating sounds or music. Sometimes that's good in the sense that we develop a unique sound, but other times it's good to introduce a new perspective.