D5100 LCD unusable for manual shooting...should I care?

The problem that color constancy causes is that we see shaded areas as being much brighter than they really are.  So when you look at a scene with sunlit and shaded areas, you simply have to know that the shaded areas are darker than you see.  There can be a three, even 4 stop difference between sunlit and shaded areas...but to our eyes they look like a one or two stop difference.

In this case you simply have to know that the shaded areas are going to appear too dark in your image, so you have to do something about it.  A person may decide to use a GND filter to balance the lighting, use fill flash, or overexpose the scene and fix it later (provided that the highlight areas have enough headroom to do that.)  Another option is to simply recompose to cut out the shaded or sunlit areas of the scene.  So it doesn't matter how much you check the scene with a meter...there's simply no exposure that's going to get you the scene as you see it.

All the consumer bodies have exposure preview when in auto modes.  If you increase or decrease exposure compensation, the LCD will get brighter or darker.  What you see in the LCD is usually what you get.

A P7700 weighs 14 oz and a D5100 - 20 oz with no lens.  Kit lens is 9 oz. making the D5100 twice as heavy as a P7700.  My 18-105mm (kit lens for my D90) is 15 oz., so it weighs as much as the P7700 itself.  My 70-300mm, a common second lens, weighs 26 oz just by itself...nearly twice a P7700.  So I'd say you were a little off the mark on that one.

I meant that when you look at a scene with sunlit and shaded areas, the difference in lighting between sun and shade will appear to be much greater on the LCD.  Many people think this is a dynamic range problem...but it isn't.  The LCD is accurate...it's your view of the scene that's not accurate due to color constancy.  So people end up messing with exposure settings trying to get the LCD to match what they see...but that's never going to happen.