Why 2 pole AC power switch?
The purpose is to isolate the circuit such that no voltage is present on the live or neutral to make it safe for maintenance etc.. As @Incnis Mrsi says you can't take the potential on neutral for granted, the system may have earth sat between live and neutral (where earth may be to a centre tap on the substation transformer) or there may just be significant volts between neutral and earth where you are on the circuit (this is common where the supply doesn't provide an earth).

It is worth noting that for these reasons it is quite possible to trip an incoming RCD if you touch neutral.