The proper set-up isn't that expensive. Vital circuits are placed on a second panel with a switch which will power it either from the main panel or your generator. Putting power back into the lines is, indeed hazardous, but if you have your generator connected to the 'line', there is the further risk of having a VERY bad day if the power comes back on. You are working with alternating current, changing from positive to negative and back 60 times/second. All current sources on the grid have to be in phase, that is positive or negative at the same instant. If not...well, we've all seen the sparks that result when you hook a couple of 12 volt batteries up incorrectly. Just imagine that at 220volts. Things go flash and bang and you have to spend money on something besides the P-car.
Play safe
Les