Long line trolling does work all year, but in different forms. Most of the summer on the Mississippi my trolling is made up of regular crankbaits without boards on channel edges, and flats. The big thing that makes a difference throughout the different seasons is crank styles and speed.
For example, late fall is one of the best time to catch tons of fish trolling, (mid to late November and later) but they are usually deeper and want a slow moving bait around 1mph to 1-1/2mph. This is where leadcore and three-way rigs shine.
I personally prefer leadcore as I still get that “Long Lining” feel. From now until about 45 degree water, I’ll troll a lot of jointed Shad Raps, Reef Runners etc… in the 10-16′ depth range.
For clear lakes, sometimes the boards are better. I prefer to long line whenever possible though, since it’s one less thing to worry about. I don’t have to worry about taking a board off, or having a line break on a snag with a $25 board (with an additional accessories) on it.