?
If we are in a corridor and all the bad guys are lined up perfectly for a lightning bolt, a line that starts with you can not be used if the bad guys are meleeing your allies in front of you, while a ranged explosion that starts past them can if there is enough room.
A line can be used in smaller spaces where a 20 foot radius AOE cannot be used safely, but you have to have room and the ability to maneuver to get your allies out from between you and the line of fire with your target.
That is an intensely specific example... here's another one.
Your group's fighter and barbarian are fighting a guy on the battlefield. 80 feet away, and in the air, a Dragon is closing in.
Using Lightning Bolt, you can stand right next to the guy on the battlefield and draw a line to the flying dragon without hitting any of your party members. A Fireball can hit either that one dude or the dragon.
Here's another one: You're on a battlefield scenario. There's a messload of orcs and goblins and allies all kind of in a big checkerboard scrum on the field of battle. You could use your fireball on the edge of battle and get maybe 2-3 goblins or orcs without hitting your allies.
Or you could do a nice line through the middle of the scrum, picking the line that avoids hitting your allies, and hit 5-12 dudes based on the specific layout of the battle.
Or, y'know, swap places with the fighter types and blast down the hallway. Or spider-climb up the wall to stand on the ceiling in the square above your fighter's square and lightning bolt down the hallway in front of the fighter.
Lines have a place where they can be more effective than a Fireball. And that place is usually when the chaotic melee of battle makes it hard to avoid hitting allies with spells.
That said I'm still gonna do the zig-zaggy lightning bolts, from now on. Maybe have each segment of the "Jump to a target that hasn't been hit" have a minimum length of 10 feet (For range purposes) and can't cross squares the lightning bolt has traveled through... FUN!
