Well, the D&D spell I presented would allow a humanoid to assume the form of other humanoids (Humans, Elves, Dwarves, etc..). Just that is fairly powerful, as there are a few humanoids with wings or the ability to breath underwater (Which can be useful).
With the ability to change small aspects of your form, it creates a much more potent and versatile effect.
Look at it this way. The same level you get this, someone else gets the ability to light their sword on fire. Just that.
And you can use your ability to make skeleton keys, escape bonds, thicken skin, create small weaponry, and disguise yourself.. all rolled up in one.
Which is why Marv is objecting.
Just thought you may want to know.
“She'd become a governess. It was one of the few jobs a known lady could do. And she'd taken to it well. She'd sworn that if she did indeed ever find herself dancing on rooftops with chimney sweeps she'd beat herself to death with her own umbrella.”
― Hogfather