We need a comprehensive professional who possess both sets of skills. It is not very helpful to have a “UX” person who can talk strategy, data, etc but cannot translate that understanding into a beautiful “UI”. In my experience, we have good supply of people (who focus on analysing user data, competitors, strategy. etc). Similarly we have great supply of people with traditional visual design skills (who focus on layout, color, typography, etc.). But we do not have many people who can intuitively convert user data into an interface (or experience) that users are going to desire or love. And we do not have many people who can take a leap from beautiful layouts and typography into an interface that’s learnable, memorable, desirable, removes friction, improves productivity, increases sign-ups, communicates brand proposition, etc.
In other words, there is always a “loss in translation” between UX and UI people, or between strategy, user understanding and the actual UI.
A professional who is adept at both aspects and can swiftly glide between the two, has greater odds of creating successful interfaces/experiences.