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

When I'm shooting, I'm more worried about the composition and I'm looking through my viewfinder rather than at the LCD for a live histogram.  I think most DSLR users are the same way.  My camera meters just fine and shooting RAW I pretty much have +/- 3 stops of ability to adjust highlights and shadows.  If you're using the brightness of an LCD to tell you anything on a modern camera that's just crazy.  Take photos, review them on the LCD for composition, look at the histogram to see if you want to make changes.

If you're looking for a camera to stare at a live histogram instead of focusing on what is important might I suggest something with an EVF.