You make a very interesting point, Joe. On the face of things, a pellicle mirror has obvious advantages: no mirror slap, no mechanical movement to waste battery power and wear out and blow the dust around, frame rate limited only by the shutter iteself and (of course) the electronics, live view very easy to achieve .... there is a lot to be said for it.
One imagines that it is (relatively speaking) a simple task. (Surely it's easier to make a 50% reflective mirror than it is to do the near-impossible they already do with moving mirrors.)
On first sight, the cost seems excessive: only half as much light falls on the sensor, and slightly more than half of the available light goes to the viewfinder. (Less than a 50% drop to the VF because we already lose a bit because the existing mirror isn't 100% efficient. Too small a difference to worry about though.)
But half as much light is only one stop. Thinking about the viewfinder first, sure, your lenses would all be darker, but a single stop would be quite bearable in most cases: a 50mm f/1.8 would be as bright as an f/2.5 lens is through your current camera, a 24-105 f/4 would "look like" an f/5.6 unit. But using something like a 400/5.6 with a 1.4 converter (f/8 native) would be pretty dark at an effective f/11. Still, the advantages are probably worth it.
But there is a second factor to consider: focus. Auto-focus systems love light. Would we really want to live with a 1 stop degradation in our auto-focus? It make a very noticable difference. For example, when I put a 1.4 converter on my 500/4 (making it effectively a 700/5.6) it takes about 60% longer to focus. I'm not sure I would want to use a camera that did that to all my lenses, all the time.
And finally, there is the loss of 50% of the light hitting the sensor. Again, it's only one stop, but that turns your 50mm f/1.4 into an f/2, your 100mm f/2.8 into an f/4, your 400 f/5.6 into an f/8 .... pretty scary stuff. Better and better high ISO performance will help, of course, but then we all want to press that advance into service to shoot in lower light or gain faster shutter speeds.
So: some big trade-offs. On the whole, I think I'd rather stay with the mechanical mirror. But it may be different for you, and different again for the next person. It would be nice to have the choice. Will any of the SLR makers offer it to us anytime soon?
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