A steel frame is not necessarily so great. If you're buying a new, boutique one with high-end steel (say a Waterford) maybe you don't need carbon bits. But an old 70's bike can be nicely improved. I converted a Zebra to a threadless carbon fork, put an Easton seatpost on it (and yes, made it a fixed gear), and it made it a smooth-as-glass bike that handled better because of the fork rake and shorter wheelbase. But I also have a 725 steel Fuji Connoisseur build that has a straight steel fork, and it rides great. I put a carbon post on it because I've found it's an easy way to dampen the ride & drop a few ounces.