I'd be in big trouble without my fast, full sinking line. I fish deep lakes and during the cold weather, the trout hug the bottom. I use three lines for stillwaters and doubt if that will ever change. I catch fish so the approach works for me. I always carry a floating, a clear intermediate sinking and a full fast sinking line. I'm not keen on the "bobber fishing" (vertical) and prefer using sinking lines instead. However, if push comes to shove, I can and will switch to a dry line, weighted patterns and very long leaders. Probably the move valuable tool I use for flyfishing stillwaters is my portable depth/fish finder. The screen indicates the depth where the trout are holding and this information is instrumental in my line choice. About this time of year, the fish normally hold within a foot or so of the bottom of the lake. ... but no always. That's where the depth/fish finder is so danged handy. I may not catch the fish I mark but at least I know the depth they are holding so I can choose my line and presentation accordingly.