I think the writers wanted Buffy to have a human boyfriend, to give her someone who was a little more safe and wholesome. I personally loved Riley... People call him boring, and I get that when he's compared to Angel and Spike, but I think out of all three of the love interests she had on the show, he was the best one for her because they were both human and could have actually built a life together. I thought SMG and Marc had good chemistry, and their relationship was sweet up until the end. I actually hate that they wrote in the whole cheating with vampires thing, and if they were going to write Riley out of the show they should have just cut that and let him go back to the military without the cheating storyline 'cause it was completely out of character for Riley. I think they realized most fans hated Riley, so they wanted to get rid of him as quickly as possible so they could make Spike the new love interest. But overall, I totally get the writers' love for Riley, 'cause I felt it too.