If you can taste that your fleur de sel is the ocean in a grain of salt, you don't want that lost in the shuffle. But if you get much the same flavor, or an equally exciting one, from another "gourmet" salt, the two can be used, if not interchangeably, then at least competitively. For example, I find that Hawaiian black and pink salts have a peculiar oceany flavor that is quite different from fleur de sel, and in my experience they are also very dry and hard, giving a very different sort of texture. For reasons I can't entirely explain, something about Hawaiian pink salt reminds me of flavors I loved in Kyoto, so when I make Japanese food that needs finishing with salt, I finish with the pink stuff, and I find fleur de sel somehow peculiar in this context -- I don't know how to explain it, but somehow fleur de sel seems slightly squishy, perhaps because the sorts of Japanese dishes that need finishing with salt are the ones that are not themselves squishy, which is rare enough that you don't want to undermine it with a moist salt. Does that make sense?
Anyway, it's in my opinion something to play with. As Shroomgirl notes, a good deal of what gets sold in the supermarket as fleur de sel isn't, quite, but on the other hand I think it's close enough that you can fool around and learn something -- and the price means you can do it with impunity.
For what it's worth, Julia Child never accepted any of this stuff about salt. She used iodized granulated salt exclusively. When asked by Alex Prud'homme about this, about gourmets who insist on kosher salt or fleur de sel or whatever, she said gaily, "f**k 'em!"
Good old Julia!