So the IMEI number is basically a unique serial number to the phone. It's like an ethernet MAC address. It's supposed to be unique. If someone got ahold of yours, the worst they could do is pretend to be your phone's serial number. They couldn't access your account, they couldn't use your data, or make phone calls as if they were you. All of the account information is on the SIM. There are a few reasons why someone might want to spoof an IMEI - despite the fact that it's illegal - such as to avoid using a phone with a stolen IMEI, or to pretend that one phone is another (such as making an iPhone look like it's a Nokia to avoid specific iPhone charges/plans).
But I don't see that it's a particularly important thing for someone to be careful that no one else knows it. It's written on the back of a lot of phones, and it's easy to find under the battery on nearly all phones. It's printed on the outside of most cell phone boxes. If someone wants an IMEI, they can just write down the numbers on boxes.
http://en.wikipedia.org/wiki/IMEI