Fluorocarbon as main line
Fluorocarbon is almost always used as a leader, and price is the usual explanation. Price is not the main one: the real reason is how it behaves on the spool.
Stiffness against a fixed spool
On a spinning reel the spool does not turn, and the line comes off it in coils over the lip. Each coil has to straighten in flight — and fluorocarbon is stiffer than nylon and straightens less readily.
Everything else follows: more memory, more thrown loops, a shorter cast. The very stiffness that makes it a good leader keeps it from being a main line on this type of reel.
Where it does work as a main line
Where the spool turns by itself. On a baitcasting reel the line comes off in the wraps it was laid in, no coil has to straighten, and the stiffness stops mattering.
The second place is short range in general: where there is no cast to speak of, or it is a few metres, the drawback has no time to show.
What switching to it buys
The same things it is used as a leader for, only over the whole length: it sinks, it is hard to see in water, and it minds rubbing on stone and shell less. For a bottom retrieve that means direct contact with the lure, without the surface bow nylon holds.
The properties themselves are covered in the entry on fluorocarbon.
The middle road, which most people take
Instead of filling a spool with fluorocarbon, they run braid as the main line and a long fluorocarbon end section — several metres or more. Soft line then sits on the spool while fluorocarbon does the work in the water.
That is why «fluorocarbon main» is rare: nearly all of its benefit is available without it. The long end is tied with a flat join, as a shock leader is.
What has to be paid for it
Besides the behaviour on the spool — knots. Fluorocarbon holds knots designed for nylon less well, and more turns are usually recommended on it. The basis is the same as everywhere: no knot holds as much as the unknotted line.
What cold does to it
Fluorocarbon stiffens further in the cold, and what is merely noticeable in summer becomes decisive in winter: the coils leave the spool grudgingly and the cast shortens further still.
So it is not taken as a main line in winter even where it worked in summer. As a leader it is: a short length is not spoiled by stiffness. What else winter changes is set out in how to choose line.
What we do not state here
By how much the cast shortens, or by what percentage fluorocarbon is stiffer. Both figures circulate in reviews, and we found no verified source for them. The difference shows on the water in a single session — that is the check that needs no number.