Lets try and keep things in perspective.
Why is the fire extinguisher even there?
Due to certification and current material requirements your aircraft should not burn (very low probability i.e. the least of your problems).
Now what type of fire are you going to attack with a little tin pot extinguisher?
I have fought an internal engine fire (Garrett TPE 331) and 3 of the Halon extinguishers I used didn't do much.
More than likely, something introduced into the aircraft that is flammable, will be the problem. DG regulations are supposed to cover that.
In some countries with less than 9 pax the extinguisher is optional. The impetus is on not putting anything flammable in the aircraft. It is up to you if you wish to fit one as it may be of additional assistance to an incident outside of your aircraft. In fact it would probably save more lives if it was fitted to you car!
Its a bit like RFFS - hell of an expense for very little return. With a helicopter think of how many times you operate where RFFS could actually get to you.
A DP extinguisher satisfies the requirements in some countries but is not actually much good at putting out fires. The installation of a cabin fire extinguisher is mandated by NAA's and not certification requirements.