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

You presume incorrectly, and as someone who doesn't know anything about photography, you sure are making a lot of assertions on how a camera should work.  So lets get you straightened out.

First of all, if you expect to hold up a large DSLR 12" from your face all day long, you better have a good gym nearby 'cause you're going to have to beef up your arms.  and not only is it a problem to hold the camera up, but holding it steady is troublesome as well.  With the viewfinder at your eye, you can press the camera into your face, and your elbows into your body to create a very stable platform for your camera.

Second, I wish I had a nickel for every post from someone who didn't understand why his image was overexposed/underexposed because it looked correct in the LCD.  The bottom line is that those LCDs on the back of cameras aren't good enough to judge exposure or color.  There's no calibration process available for those LCDs.

Third, the histograms from image review are already fairly useless...live histograms are even more so.  Here's a correctly exposed image and its histogram...it's correctly exposed because increasing exposure by even 1/3rd stop would cause clipping of the red channel.  There is no way to have set exposure correctly using the histogram.  The highlights barely register at all.  In this case, exposure was set using a gray card.

This image also illustrates the biggest problem with histograms...you need to know where the tones of your scene should be, in order to know if your exposure is correct.  but if you know where the tones should be, then you can expose them correctly without the histogram.  That's why the histogram is useless.

If you just want to take pictures without having any photographic knowledge, then just buy a point-n-shoot.  There's nothing wrong with that.  But don't disparage Nikon just because they make cameras for photographers.