You know, around here is you wave an AmEx around, you're likely to be asked to produce a REAL card...
In all honesty, it is better to have one of each(at least Visa and MC). I have been to a lot of places that don't accept AMEX, and a handful of ones that will only accept one of the other two. An example would be on my recent flight back from Cozumel when I upgraded to 1st class.