Okay this idea just popped in my mind and I was really curious if anyone has ever written something of the sort sometime: Spike seducing a villain for saving-the-world or whatever other purposes. Maybe he doesn't do it on purpose at first but then uses the advantage? I don't know, just tell me if it even vaguely reminds you of some story.

Oh I would love to read something like this... I have thought of that too at some point.
Anything?

Well these are the two fics it made me think of although I'm not sure either are a great match for what you probably want...

Spike has a very brief 'other' in Anticipation as part of retaining his cover if that would count. But it is very brief and, as I remember, more mentioned than particularly seen. :s

Anticipation by 2writers4spike
Having no idea how it happened, Spike finds himself back in 1977, reeling from a newly acquired soul and more guilt than most could handle. After dodging Drusilla, his main focus is getting back to his own time and back to Buffy in Sunnydale so that he can atone for all he's done.That is, until he sees the newly born slayer in 1981 and goes off in a new direction. Can he stop the events that brought misery to Buffy's life, or will he be forced to watch destiny destroy the woman he loves all over again?

Spike kind of uses the advantage of Faith's interest in him in this fic but it is much more forced on him than you probably mean...

Lying Eyes by dreamsofspike
AU Post Season 2. The events of "Anne" never happened and Buffy did not go back to Sunnydale...until almost a year later when she finds out things aren't going so well at home....there's a new Big Bad running Sunnydale, with a certain blonde vampire at her side...Buffy returns home to help take down this new threat...but which side is Spike really on?