Single pole switches the Live
Double pole switches the Live and Neutral
The use of double pole switching guards against the dangers associated with a Live/Neutral reversal. In this circumstance a single pole would break the circuit on the neutral leg and the device would not operate HOWEVER lurking within would be the Live and the associated danger of electrocution. If both Live and Neutral are switched then this possibility is removed.
In summary so long as everything is wired correctly single pole will suffice but to guard against a Live/Neutral reversal double pole switching will always ensure safe disconnection.