Poster designers are concerned with catching the public's eye and delivering the necessary information, but catching the public's eye is arguably more important. Other designers exclusively do branding, which is all about 'look' and hardly about 'function'.

In reply to the original point, I would say that, on one hand, how something looks is part of how it works, if you consider the entire user experience as the function of the product.

On the other hand, I think what you're saying is that you're reluctant to hire designers who only do a particular style well. I agree that a good designer should aspire to broaden their pallet, but I see no reason to condemn those who for whatever reason are not capable of or interested in that.

For comparison, consider that some fine arts produce work in the same style for their entire career (i.e. James Turrell) while others explore a broad territory (i.e. Gerhard Richter).