Shelling a solid model (or creating an offset surface, which I believe uses the same calculation) is often impossible. There might be a small area of the outside surface that has a curvature such that it would be inverted if it were offset to the specified thickness. Consequently it is necessary to laboriously create part of the surface manually to patch this area, (or change the design).
Now: this manually created surface is of course different to the one that would be created by the shell command, were it successful. But, it usually (IME) turns out to be quite adequate to the needs of the manufacturer. The surface is not perfect, but it so close to tolerance that it results in a good wall section with no sink marks etc (talking about injection molding, die casting, thixo molding, etc). In other words, if the nominal wall section is 3 mm, this manually created surface might result in a wall section that in some places is, say, 3.05mm and in others 2.95mm.
Another way to interpret this is, that the level of precision with which the shell command and the offset surface command calculate the offset surface is way too precise for the purposes of 99% of manufacuring processes. Perhaps if you were designing a lens it would more important.
Why not build into the shell command a (for want of a better term) fudge factor, that allows some lattitude in the created of the shell within certain limits set in advance by the modeler. It could be an instruction to the effect of, "offset this surface 3mm plus or minus 0.05mm if needed".
I realize there is an Approximate Offset built into the Offset Surface Command (but for some reason not in the Shell Command), but in my experience this has been useless, i.e., it is way too approximate. An offset of 2.0mm approximate might result in the wall in some places being 3.0mm. This is a little too approximate, but the principle is the similar to what I am suggesting.
tl;dr Shell and Offset Surface commands should have greater flexibilty to deviate from the nominal offset value when necessary, within a certain tolerance set by the user.