Ignoring RCD's, GFCI's etc for the moment, as many domestic installations still don't have them, then...
Earth should only ever carry current in a fault condition. Neutral, on the other hand, is a current carrying conductor. It has flowing through it the complete load current.
If there is any voltage drop (and basic oms law will show that there always be some, more if there are bad connections or undersized wiring) and you connect earth to it at the outlet, you are essentially raising the potential of any grounded chassis with respect to any other grounded item nearby that is not powered or is earthed "incidentally", such as a steel sink via metal plumbing. This "small" potential difference has very small source impedance and under the right conditions can cause sufficient current flow to cause death or fire