i think its normal for an 18month old not to want to have one. some kids go through stages where they wont have them and then go back to them later. DS was never a fantastic day sleeper, but will generally have one as long as DH or I arent the ones asking him to, so daycare or people babysitting hes fine with.
I think at that age its still impotant that they get some sort of rest in the day even if they wont sleep.