Lightning strike straight out sucks, pure and simple.
The only reason to ever use LS is- if you don't have spammable FL.
Which means 1- for some reason you've gone full lightning (I dunno, maybe that's good for pve- I don't do operations so I dunno... full lightning however is absolutely terrible for pvp though so never go it). or 2- you're a corruption/lightning hybrid.
I am the latter- no spammable FL, and LS gives me CL procs as well as my force regen- it's also my only spammable dps ability.
Let's say though I was a spec that had wrath and CL procs (lightning/madness hybrid, presumably for the root KB and stun bubble)- in such a case, I would indeed spam FL over LS, not even a question- wrath procs, force regen, snare, far better damage, only reason I'd use LS was for a wrath proc where CD was on cooldown... that said, I dunno if I'd go such a hybrid- yes, it has some better control, but to be perfectly honest lightning doesn't add much damage at low levels... LS is terrible no matter what, CL is shoddy damage even when instant proc- and it's never worth the 3 second cast.
Best thing low other than the CC in lightning is lightning barrage procs for quick FL.... so it'd come down to that and the CC vs creeping terror's root and the extra dot damage high in madness.
R- there is never, ever, ever a time where I would spam LS if I had the spammable FL talent (along with wrath procs and the force regen from it).