To make sub-trees more appealing (after capping), you need to have meaningful choices in trees other than your primary, that are still useful to said tree as well. For instance- shields in lightning. A healer drools over this talent, as it's amazing for them, so they're encouraged to go down that tree to pick it up. However, if they do, then they won't be able to pick up, say, Oppressing Force from Madness that would give them more utility within the group. Thus, it's an interesting choice.
Force Lightning is the most "DoT"-y of the lightning spells. Since a DoT heavy class/spec still needs to have a filler (how boring would it be to cast 2-3 DoTs, wait for them to fade, and re-cast?), it makes perfect since to have Force Lightning as the filler for the DoT tree, as it does it's damage over time.