I can see many reasons why people ships them. To be honest, I have no problem with this ship. Come on, it's cute.

Sunset Shimmer and Slash have many simliarity; they both used to be evil, doing destruction everywhere they went under the influence of greed and ego; and they both had turned over a new leaf as they try to seek a life where they could put their past behind.

And before they became evil, they had someone close; someone they could always rely on ( Sunset with Princess Celestia and Slash with Raph ) but their ruthless action has caused them to lose the one they secretly care for, despite everything.

Come to think of it, they both has a leadership skills within themselves and this is proved with Slash as the leader of his group and Sunset who acts like one even after Sci-twi comes in. They both also pretty much snappy, with an unpredictable temper and explode in certain times.

I'll give this ship 10/10