When somebody says "I need my device that I carry with me at all times to connect to the company's mail server", they're saying "I want to do more job more efficiently."
Yes, and they think that's the best way - but they're also not solution architects.
However, the IT guy isn't denying things for shits and giggles. His job is to make sure the entire infrastructure stays up, secure, and available to everyone.
If he allows every Tom, Dick, and VP of Marketing to connect their new shiny to the network without doing his due diligence, who do you think is going to have his balls in a vise when that device goes insane and screws with the infrastructure? Not the VP of Marketing, that's for damn sure.
It's a balance. Everyone wants their new shiny, but they can't always have it. The IT guy wants a simple monoculture, but he can't have that.