While the risks, assuming you're doing this with some static protection, are fairly small, they are real. Note that when you remove the shell, the board is already connected to all the external circuitry, and thus there is a low impedance path for any current to flow. However, a bare board has all the circuitry just waiting for some current to flow in the I/O circuits, you don't want that to be a large static discharge.
As for the carpets, there are a lot more things that create static, trust me, I know. I have a wood floor in the workshop, but I can still draw a nice arc to the steel legs of the workbench, turns out the fabric on the chairs generates the static as well. Synthetic clothing will also generate static quite well.
The dangers are not huge, but they do exist.
