They're great for collectors and those who like to do cardistry (card flourishing/acrobatics).
Sometimes there's a certain theme to a performance that a performer wants to convey. A performer's vibe can determine the environment the audience feels like he/she is in. Some of these cards would be useful for that purpose.
Other times, it's just to catch other people's eyes. "Oh that's just a regular deck of... wait a minute... what kind of design is that??"
As for magic, I've never ran into any problem with the audience thinking they're gaffed. However, I introduce them in a way that takes away the suspicion. I pull them out and just stand nearby, doing some basic cardistry moves. I might escalate the complexity of the flourishes if I want to show off.
In most cases, people will approach me and ask if I do magic and my response is, "I can," as in 'it's not my main purpose but I can show you some if you like to see'. That usually takes away the suspicion, especially if I follow up with doing cardistry for my own entertainment.
If you're unsure of what to think about vanity cards, consider looking up Jackson Robinson's Federal 52 series of playing cards. They're Casino Bee quality with artwork influenced by the American currency. Highly successful and highly praised so much so that he's been hired by USPCC if I remember correctly.