Kinda hard to get a proper exposure let alone take a freaking picture without a lens or the cap on... But what do i know, it may be the new big thing in photography... photos with no lenses.
A light leak somewhere in the body would be an issue for those of us who do shoot in darkened environments like museums without tripods (some museums don't allow tripods), or night-time photography, in that while there are times we can take the time to make sure we get that shot, there are other times where we can't take a couple minutes to get the shot right, and have to take it "on the fly".
This is where the camera's automatic metering comes into play; and if there's a light leak that throws this into doubt...
But on the fly, i'm assuming you would be framing up your shot with the VF or rear LCD and not having the top LCD shining away.... I guess to me it was a nice reference but something i rarely used to base exposure off of. Every so often I shoot for banks who send me out to take photos of buildings they have mortgages on, like retail centers/warehouses/apartments... If the buildings are occupied, great, but other times i'm shooting in buildings, sometimes without power turned on, pretty dark.... Shot with the 5d2 which had the same problem but it wasn't a problem... never affected me to the point where it was an issue. I can see your point for how i work, i would never base exposure off of using the top LCD.