Any extra knowledge is always a bonus, and learning .NET will increase your chances of understanding how applications use SQL as a back-end.
I have seen many job requirements for SQL Dev's, who actually require them to know a bit of .NET, but not always a fixed requirement.
Knowing .NET will seperate you from a normal SQL Developer, and it wont hurt knowing the other side....
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This thing is addressing problems that dont exist. Its solution-ism at its worst. We are dumbing down machines that are inherently superior. - Gilfoyle