You're right of course, so far it works only for NonLinearLSQFitter. I had two ideas in mind:
(1) This option was only available for Gaussian1DModel, but I thought it could be reasonable to have it for 
all models, thats why I moved this option to the Fitter class.
(2) When you're fitting data, you may want to check if the result changes, whether you take the analytical
or estimated derivative (especially when you're working in an interactive shell). Having this option in the __call__ function, you don't have to setup a new Fitter instance to change the "derivative mode". That was the only reason for putting it in the __call__ function. 

Is this reasonable? Thanks for checking the changes!