The pros are that with an OVF, an EVF, and an LCD screen, you have all three options easily available (if only the rear screen would flip up, you'd have the whole spectrum covered). The only con would be if it didn't work, but on the X100, it WORKS. Really well. I've found a couple of things since I got back into shooting a year and a half ago or so. First, I don't really like using ANY eye-level viewfinder most of the time. Coming from an old film SLR background, that surprised me a bit, but its un-deniable. A STRONG preference. I also find that when I DO want to use an eye-level finder, I much prefer a good OVF to an EVF in any situation that doesn't specifically call out for the EVF. The X100 isn't my primary camera - its a wonderful and amazing change of pace and perspective check. I love it and I love using the OVF, but I don't use it for a lot of the kinds of shooting I like to do. The only time I prefer the EVF is in low light, where it might be too dim to see well in the OVF, but the EVF will gain up so you see what you're framing. Other than that, I much prefer the OVF, particularly the X100 OVF which shows you the scene developing outside of the camera's frame. And its in real time to the extent that time is real! I love getting a very quick review in the EVF after taking a shot with the OVF - that's my most used function for the EVF. But for macros or in low light, I'm thrilled to have it. And sometimes, for shooting from or at weird angles, I just use the LCD. The X100 isn't really quick enough to be a good hip shooter, but as long as I can see the screen to frame, I've got time to shoot.
I can't think of a downside to the hybrid. I'd love the OPTION on any camera, but other tradeoffs are more important to me. I'd love to see it on an interchangeable lens camera, but I have trouble imagining how it would work with ever changing focal lengths. Note that Fuji didn't eve try with the X10, but that may have been as much about trying to hit a price point as any technical challenges they may have anticipated.
-Ray