The answer to me lies in changing the question. Replace the word "designer" with "artist" or "writer" and the ask the question again.
Would you ask a writer or an artist to confirm their style? Should they? I say no, and the reason why is simple. Creativity is a personal thing, and design is a creative endeavor. If you ask a designer to stop designing in their own style you are asking them to design something that is outside of themselves. To design an impersonal thing. It would be like asking a horror writer to write ad copy. It's just… Wrong.
A brief addendum: you can always ask a creative person to "stretch", to create something outside of their comfort zone or in a medium they ave never used before… That's fine. Many designers will embrace such an opportunity but the designer's personal style will still shine through. That is the result of us being individuals…
Of being human.