I just did a similar test, taking two outdoor pictures with my new 5D MK iii. The one without the lcd light on is metered for 10" exposure duration and turns out fine. When I use the lcd backlight the meter shows 6" exposure duration and the shot gets underexposed by almost one 1 stop.
I acknowledge these tripod mounted night shots with long exposure duration are not the ones I do regularly, but to me the light leak problem seems very real.
So, should I send the camera back?