OK, here's the thing: Mark likes complex, complicated, morally grey characters. He doesn't mind what they do, or how unlikeable they might be, as long as they make sense.
He flips out into ALL CAPSLOCKS when it comes to unexpected plot developments, but just look at his posts on Five by Five, Sanctuary and The Yoko Factor - careful and considered, and barely a capital letter in sight. No ranting against, say, Buffy (who's plenty unlikeable in Sanctuary), or Angel or Spike later on. He found TYF uncomfortable as hell, but admired how clever it was. (Also see his reviews of Torchwood: Children of Earth - thoughtful, appreciative, full of admiration.)
So here's my prediction: Fool For Love will happen, and a) He won't see it coming (because he's not very imaginative in that sense) and b) William the Bloody Awful Poet will hit him like a two ton truck and he'll never recover. Remember what we know: Mark was very badly bullied [for not being masculine enough/being gay], and I cannot imagine that he won't identify with Spike, esp considering Spike's continual outsider status. He already likes Spike for the snark etc, but knowing that a lot of it is a front, armour to protect himself/make him fit in, as well as the way he'll go on to, continually, lay his heart at Buffy's feet... ALL THE FEELS YOU GUYS! *g* ("You're beneath me" <- Mark is NOT prepared.)
How it'll play out over the long run I'm not sure, but I think he might just love S6.