It really depends on what you do with the points and how valuable they are to you. Marriott sells points (50,000 per year) for $ .0125, so that's what I use to determine value. If I want to stay at a Marriott I compare the cash price to points at the .0125 price. If it's cheaper using $$ I'll save my points for another trip.
Of course, this will change with the new point structure Marriott announced for Jan. 15, 2009. I might use my AmEX card more now.