... but don't share it for imessage and face/time or icloud
You can share the ID for iMessage and iCloud, no problem. In fact, some people prefer it that way to get messages across multiple devices and/or get iCloud e-mail across multiple devices and/or share a photo stream without having to manually Share a photo stream.
The only problem with using the same ID for iMessage and FaceTime is if different people use the devices. In that case, all one has to do is de-select the ID in the "Receive at..." setting and designate a different e-mail address.