I find that line infantry are mostly good for...well...being line infantry. They stand in a line and shoot things.

Typically 3-4 lines is best for firing. 3 lines prior to fire-by-rank since you need that extra volume to get a decent kill rate. After fire-by-rank, 4 lines allows the front ranks replacements when people get killed.

I also find that there is very little that can budge standing line infantry in defensive mode for melee. As long as the enemy comes from the front, they're probably not going anywhere.

Once again, 'hammer and anvil' tactics are the best and simplest method of dealing with most forces. Tie them up with a group of line infantry and hit them from behind with cavalry. %80 of the time you get an instaroute. If you can time it right, and have your infantry and cavalry hit at the same time...well, it's glorious ;)