Ironically, such clients also seem to expect that whatever additions/changes they dream up should be able to be folded in to the plan willy-nilly. Whereas if they accepted an iterative process that would come naturally, without constant re-negotiation or ill will.
They can! That's exactly the point of agile and is how you get them onboard. When clients want a feature that is weird or complicated or whatever, the answer is never "no" the answer is "sure, now tell us where it fits in the priority list".
