customers insisting on fondant look but no fondant

You just put your foot down. You're the expert, and you explain to them that you know what you're doing, and the design must be done in fondant. People and their ridiculous aversion to fondant crack me up. I put just as much buttercream underneath the fondant as would be on a BC only cake, so it's not like people lose out on getting icing. 

Just check out Cakewrecks sometime, and see the posts that compare what someone ordered to what they actually received. It's pretty easy to see the ones that should have been done in fondant, but the baker was to ambitious or got strong armed into doing it in bc.

"Some cakes MUST be done in fondant to look like the picture.  Your choices are to either have the cake covered in fondant or choose another design."
 

Just tell tham that you can smooth buttercream out, but it will never look the same as fondant. If they don't mind that then a lot of designs can be done with buttercream and fondant accents. If it's something that CAN'T be done with buttercream at all (but I can't think of a lot that can't) tell them. As long as they're aware that it won't look as smooth and they're okay with that then go ahead. What designs are they talking about?

Also, if you don't think that YOU can do it without fondant, that has to be the deciding factor. I do about 98% buttercream and I decorate them with the same kinds of things I'd do for fondant, but I've been doing this for years, so I know a lot of tricks. If you're not comfortable doing a design you could suggest alternatives to them, but never tell someone that you can do it if you're not sure!

Or learn to use rolled buttercream.

I have to agree with AZ, cathy, and czar. 

Sometimes, there's a need educate the client and to stand by the education. 

It's really OK to say, "I'll only do that design using fondant.  Done in BC, neither of us will be happy with the results."  If the client doesn't like the answer, they can choose another design or another baker.

As for rolled buttercream................I find it tooth achingly sweet, greasy, gritty, and although it's self-healing with the heat of your hand, it's a PIA to work with in large quantities.  I don't use it much anymore.

Modeling chocolate is a great alternative, but pricey.  Mixing it 50/50 or 30/70 with fondant gives you a lovely, workable product, too, but you have to factor in that extra cost.

Rae

Though there are some skilled bakers who can make buttercream look like magic, Glass Slipper Gourmet comes to mind, but you're right, fondant never looks like BC. I am always clear about my limitations when working with BC,  but I also emphasize the benefits so that they will be convinced that going with another baker would be a ghastly mistake...