I think it really depends on where you are fishing.
I use a extra fast-sinking line
for most of my flyfishing, but that's because I mostly fish high current areas
that are fairly deep. On flats, this line would be dragging the bottom so
I switch to a 10 ft. slow sinking tip. At slack tide on shallow flats, the sink
tip drags bottom with even a moderate retrieve. (BTW, sometimes it's actually
a good thing to drag the fly on the bottom, especially if sand eels are about,
although it's hard to avoid picking up weed). I know guys who do nothing but
flats fishing in slow water with a floating line and do well. They adjust the
depth to a small extent by varying retrieve and mending line. I do very little
of this type of fishing but may try bringing along a floating line for this
purpose when/if I do.
-bd