He's a well described character that helps a younger audience immediately recognise him as an antagonist. As you say, it is a strong image.
It also plays to the 'don't judge a book by it's cover' theme towards the end of the series.
I think the films visually portrayed Snape as well as they could. They took what was on the page and made it real.
My issue with any changes to an author's work where character descriptions exist, is where do you draw the line? I can see why prosthetics weren't used for Snape on his nose and teeth to make it 100% book accurate. If they had done that he would have been less menacing and less real as a character in my opinion. In the book it was fine, but on screen he would have been a caricature.
So where do you draw the line?
You could remove the ginger from the Weasleys if you wanted to when adaptating the books. Them being ginger is no more important to the story than how Snape looks; it's only a visual cue. You could use a different one or some other characteristic that couldn't be used in books, like an accent or mannerism particular to the whole family.
But why would you?
How does the change you are making serve the story and help bring the characters to life on the screen?
It is one thing to adapt an appearance to bring it from the page to the screen in a realistic way, but it's another to unnecessarily erase the defining characteristics of a character.
