The one thing that makes Sorin really good for Vampires, and really why he was created in the first place I think, is the -3 ability: Target opponent’s life total becomes 10. This is important since key vampire cards have abilities that activate at 10 life, like Bloodghast’s Haste, or deactivate as with Vampire Lacerator which makes you lose 1 life each upkeep unless your opponent is at 10 or less life. But Sorin rarely saw Constructed play back when he was in Standard. These two abilities always seemed enough of a reason to me to run a copy or two.
His -7 or so-called ultimate ability is sort of nice, too. You get to take over your opponent’s next turn, obviously to set him up for an easy picking on your next turn.
The one major issue with Sorin Markov is actually quite obvious. He’s a 6 drop. The 3 required black really isn’t a problem, but in a deck that was once extremely competitive like R/B Vampires, Sorin was just going to muck up your hand and took up slots better used for removal. He’s a nice card, but he ended up being relegated to Vampire deck builds that could occasionally win FNM’s and other local tourneys, not top 8 at Opens or a Grand Prix.
Today, though, he’s still a 15+ dollar card, even with his Magic 2012 reprint, a widely distributed Core Set. The main reason he remains so valuable is how nasty his -3 ability is in Commander. Taking a player from potentially 40 life (or even more) down to 10 can put said player in kill range. Since Sorin begins with 4 loyalty, it’s not impossible that Sorin can do this a couple of times in a Commander game while he’s on the board. With mono-black Devotion now a thing since the release of Theros, Sorin's 3 black mana symbols make him very useful for fueling Devotion effects. It took awhile for Sorin to find a home, but he’s going to be a valuable card for a very long time.