After reading many of the reviews on this website and reading the forum posts it would appear that many people are more than happy to "haggle" with taxi drivers and pay 40 Dirhams for a journey that would have cost well under 10 Dirhams if the meter were to be used.
I have been to Marrakech twice and both times I made a point of always insisting that the meter is used to calculate the fare, if this request was declined, (probably happened twice out of 30 or so journeys i made), then i would simply say thank you and walk away.
I'm intrigued as to why so many people are happy with this or think its the norm to pay at least 4 times the price for a taxi, and that's after they have "haggled"!
P.s. Dont expect the meter to be used on airport journeys.