I have a Z30 in my hands right now. The bezel is smooth, always has been. I can imagine that if one weren't careful, it could get bent and then not lay flat. I've seen posts about the back cover being damaged by people not taking care when opening it. We bought this one
after reading about that, knowing the risk (and knowing the back cover is
easily replaceable). Fortunately, you pretty much only have to open it once, to inset your SIM and SD cards, and then you're done. Somewhere someone posted, "Open it once, close it for life." That should be in the official documentation for it.
ETA: Without that notch in the bezel, the phone would likely have to be 1-2mm thicker. If the tab was a separate piece (not attached to the back cover), it would be easily lost or broken and people would be complaining about that. (I had a Bold 9000 with a tiny swing-away piece over the SD slot and never broke it, but I did know a couple of people who broke theirs hot-swapping their cards a lot and not being gentle about it.)