What does the OVF offer that makes it desireable?

the OVF can be a bit harder to use (paralax, not 100% framing etc) but I like it.  I´ve used a lot of film rangefinders (Voightlander Bessa and Hasselblad Xpan) so I´m used to the pros and cons of this kind of viewfinder and personally I prefer it-  I find it easier to compose if I can see more than just what the lens sees - it makes it easier to decide what to include or not (for me).   It´s a perosnal thing, some people hate them and will never see the appeal, some people love them.  The great thing of the X pro is that you can change it to a TTL view for the situations that rangefinder viewing breaks down - telephoto, close ups and macro, when there are things on very different image planes causing parallax issues etc etc.  Best of both worlds.