So, once more I'd appreciate your insights and thoughts to a mostly theoretical question.

I've been reading a lot, recently, about the new mirrorless APS-C cameras - Sony Compact System Camera NEX 5, for example.

The advantages of such a system is clearly in size of the camera body, of course. Disadvantage is in that you can't see what enters the lens, as you don't have a pentaprysm and a viewfinder.

(am I missing anything else?)

While I too really wanted a viewfinder, which is one of the reasons why I bought a reflex camera, I want to be the devil's advocate and ask: how important is it to have a viewfinder+LCD versus an LCD-only?

Viewfinder will show you exactly what "enters" the lens. But that same light (photons) will eventually hit the sensor. And, can't it be argued that the LCD display on the back of the mirrorless cameras will show you "what the sensor sees", which is what ultimately matters? What is wrong/inaccurate in this, in your view?