The arming of floats concept predates the immersion switches technology. Inadvertent float inflation from the automatic system is as rare as a transmission failure. Pilots inadvertently firing the manual system while constantly arming and disarming the system is quite common. What you really have are two separate systems - an auto system that is relied on to work when the belly switches sense water, and a manual system that the pilots can blow when they think they are going into water and don't want to wait for the auto system.
In my opinion, the AW139 is the only aircraft that has got it right. The rest of the manufacturers could follow suit. We have far more helicopters hit the water without inflating the floats that ones that do. Guess why? Because you have either a CFIT situation and didn't plan on going into the water or in the case of a sudden emergency there is no time to get them armed.
Our dunker/HUET training is all about exiting an inverted aircraft, so even if one or two bags are still keeping the aircraft afloat, your odds are better than if none are.