Probably your single deciding factor is going to be your target audience. If you're developing the latest, greatest MMORPG then you'll be targeting a group with upper end hardware (ps2.0+) and broadband connections. If you're developing for mass distrobution on the maximum number of PC's (for RealArcade for example) then you want to set your sights lower in terms of hardware for best effect. You can also use the "fallback" system to create multiple shaders and let the hardware pick the best one for itself. Obviously the higher end shader will "look" better, but it doesn't have to be "all or nothing."

Bottom line is this:
It depends on your cost vs. potential reward, target computer, target player, and game type.