For carp fishing, I use 12 lb. flouro line for tying my hair rigs...this is then tied on to 17 lb. Trilene XL mono via a ball bearing swivel...
The main reason is if I get a break-off it will usually be the hair rig and all I lose is my hook and not my $3 spring or method feeder...
Rarely do I tie a hair rig longer than 12 inches and seeing as a carp's teeth (grinders) are back in their throat getting bit-off is not an issue so using expensive flouro leader material would be a waste...if I was fishing for toothy critters then I would use flouro leader material...