The upside of MSoundfactory for the user is that the free version plays any 3rd party created sample library unlike Kontakt Player. So the full version of MSoundfactory is only needed from the creator of sample libraries.
Other upsides are (this is a personal opinion) the fact that MSoundfactory doesn’t need to be scripted (legato can be implemented with transition samples without scripting), creating modulations and connections within MSoundfactory is alot quicker IMO and the GUI creation is sooo much easier. MSoundfactory also imports SFZ so you can easily map any previously made SFZ format sample library there.
But still the biggest things for the USER is that you can use the free player to play any library created with MSoundfactory. And I think that would be keeping with the spirit of Pianobook. 🙂