The wife should have been shown in the film in some capacity. It's hard to take a villain too seriously when his motives are complete hearsay. The issue was only addressed by one throw-away line that dismissed the issue and shooed it off screen in hopes no one would notice.
There needs to be some kind of "proof" for the audience. Other wise, for all we know Khan is pissed because Kirk broke the head off his Darth Vader doll. For example, that's the whole point of the mind meld scene in ST09. It pulls the story together. It's not much, but it's better than nothing.
Essentially, TWOK took the ghost out of Hamlet.
Memory Alpha says that Harve Bennett wanted her to appear in the movie, but when he discovered actress Madlyn Rhue was confined to a wheelchair due to multiple sclerosis, discarded the idea.
They could've used someone else. No one would notice.