The back-emf of the relay coil has to be addressed at the relay coil. The reverse body diode of the transistor is irrelevant for this overvoltage, as it is not imposed on it directly, but trough the nearest PS capacitor and God knows how much tracks, and appears even in reverse to it, keeping it shut. Why is there such push to avoid a cheap component to address a local and well known issue, that might otherwise mitigate trough the circuit and cause who knows what and where? There are 15 relays, and while tuning they are switched over multiple times and will generate overvoltages at the coil level. The diode will solve this issue locally and any possible problems will be avoided. Otherwise, the moment 2-3-4 relays get switched at the same time, the cumulative back-emf might surprise the CPU...
Billian Marinov