I think that part of the reason they charge the rates they do (4.50 euro per kilo in some cases)is to try to claw back some extra cash in a buisness that has become very cut-throat.As a traveling musician with guitars and keyboards I sometimes have 11 pieces of luggage.Sometimes I get charged and sometimes I dont.
The last time they weighed everything my excess charge came to 450 euro.I nearly died on the spot.The check in girl crossed a few things off the list and gave me a bill to pay at another desk when I paid it she had reduced the price to 120 euro which got my heart going again.
I think that although airlines have an exessive excess baggage charge(no pun intended)its a discretionary thing and there is give and take and it is probably there to penalise the holidaymaker that brings the whole contents of his or her wardrobe on a weeks holiday rather than the person who is genuine with 11 peices of luggage
Be nice to them and they are generally nice back