You can't receive messages via iMessage without disclosing your phone-number or Apple ID e-mail address to the sender. Furthermore, doing anything that changes your phone-number (e.g. using a local SIM abroad instead of roaming) invalidates your phone-number with iMessage, but because most people use iMessage via phone-numbers it means you're forced to use roaming ($$$) when travelling, and so on and so on.

So long as iMessage is sold as "better SMS" then that's fine, as it inherits SMS's limitations (above) - but it isn't a portable, platform-agnostic, geography-neutral, messaging platform, and I'd rather people didn't try to use it as such.