That argument is akin to your dad saying that the MC5 "sound just like noise". I understand Eminem quite well if I focus on the lyrics, and I'm not a native speaker, much less did I grow up in 8 Mile.
I can't believe I'm putting up a stand here for Hip Hop, in my CD collection counting several thousand, I don't have more than a handful Hip Hop/Rap CDs. I never progressed much beyond The Sugarhill Gang, Grandmaster Flash & The Furious Five ("don't-push-me-'cause-I'm-close-to-the-eeeeeedge") and Kurtis Blow. I came to Eminem via my then 12 year old son and once I started listening a bit I was intrigued. Not enough to actually enjoy listening to him musically (the music of Hip Hop is too static for me), but enough to appreciate that it is well-executed, that he has an interesting way of rhyming and placing words, has credibility and a message lightyears beyond what others in the genre manage to project.
Besides, his mom is Kim Basinger. Which must make Alec Baldwin his father.