[Perhaps this should be a Post rather than a Question.]
As far as I can tell, RootOf is the only Maple command that takes a bound variable on input and returns an unevaluated form with the bound variable renamed (to _Z). Why is that? Either all should do it (definite integrals, limit, sum, etc.) or none should.
Making a substitution for a variable that occurs both free and bound in the same expression is a major source of programming error. Renaming bound variables helps to ameliorate that.
The unexpected appearance of _Z in their results seems to be a great source of confusion to new users. However, if more commands did this, perhaps it would be more expected.
Feel free to start a brainstorming discussion on this, as if it were a Post, even if you don't have a direct Answer for the Question. If that's the way that the thread heads, I'll change it to a Post.