No No no, that is bad bad bad.
It is close to impossible to argue against taking protective measures except with your wallet and there is also a short and long term side to that argument going both ways.
Nevertheless I'm curious to see/understand the technical justification as to why additional protection is needed. Keeping in mind also that the built-in diodes have a forward voltage of 0.3V whereas whatever external diodes you add are likely to have a significantly higher forward drop.
There's also an application note from Atmel where they suggest to use (or abuse if you like) the protective diodes for a zero-cross mains detection (one mains phase connected to a digital input pin with only a a high series resistor). In this note they state that the protective diodes are good to 1mA.
So then why should we add additional protection?