I'm with Kiltie on this one... Not replying to a "squawk ident" instruction is plain rude if nothing else... especially as 9 times out of 10 it is issued as a first exchange of transmissions between a radar unit and an aircraft.
I'd like to get a verbal response as well as the actual physical response to let me know you have heard me and we have two way comms.
Any pilot that claims they are too busy to read back the instruction should not be flying!!