What can someone do with my IMEI number?

Your question - what can someone do with my IMEI number, this is basic FUD. WhatsApp works over TCP and doesn't have anything to do with the phone network. You could run WhatsApp over Ethernet from your home network. WhatsApp only requires you to confirm the ownership of a phone number by entering a code they text you as they don't have usernames and rely on phone numbers to identify people, but once that's done, you can completely throw away your phone and use a computer for WhatsApp and it'll still work fine. Moreover, even if someone manages to intercept the number confirmation SMS (impossible with just IMEI spoofing) they will only be able to take over your phone number and receive future messages, but not read anything you've received so far. Even if the carrier's network is compromised beyond repair and the attacker is able to intercept your traffic, they won't be able to understand it as WA uses encryption to talk to their servers. Sure, at this point they would be able to intercept the number confirmation text message and take over your WA identity, but you would definitely notice (you'll just get logged out from WA) and they still won't be able to read your past messages. Any app that would upload your personal data (to a cloud for example) will do so over an encrypted connection and will use a carrier-independent username and password. They don't use the IMEI to identify you and IMEI spoofing alone doesn't even give the attacker a way to eavesdrop on your traffic (not that it'll help them much because of encryption).