git is unique in its apparent assumption that it's nothing more than a series of convenience scripts that let you work on the raw filesysten representation a bit more easily. In a way it's more a filesystem-level representation of a DAG with some sugar on top, than it is a DVCS.
Ditto. I've used Darcs, Mercurial and Bazaar, all are distributed, all have various levels of issues, but none of them required that I understand everything from the storage model up in order to actually get a grasp on them. I still have better intuition for Mercurial than Git (and long for a number of its features like GIVE ME FUCKING REVSETS ALREADY) despite not having seriously used Mercurial in years and having no idea whatsoever what goes under the CLI (and not caring a whit).
We insist, especially when hiring, that programmers know something about the internal design of other software that they use, but don't develop, on a day-to-day basis. Any programmer, depending on programming specialty, that doesn't have some idea about design of their operating system, networking stack, databases, or hardware would flunk out interview processes (and probably rightly so). The reason we insist on this specialized knowledge is because most of the time these things work fine, but when they don't the good software engineer proves their worth by being able to dig deeper.
Version control software is used just as frequently as any of this other software. But somehow there seems to be this feeling that it's not worth learning about and should just be a black box (I think package managers are treated similarly). Understanding git internals and design is a valuable software engineering lesson in and of itself.
The other aspect is having knowledge of Photoshop file formats is of limited utility unless you're an Adobe employee or Gimp developer (I don't know if the spec is open or not). Photoshop itself is closed-source and not so extensible or scriptable as an open-source command-line tool.
That's not true for git - knowing git internals reaps productivity benefits in your other work too. Treating it as a black box robs you of the knowledge to do some really cool stuff.
Broadly speaking this applies to any other software tool used by programmers - the more you know of its internal details, the more proficient you'll be in its use.
So in both respects (practical and from a learning standpoint) I think knowledge of git internals for a git-using programmer is a desirable thing.