While it's true that Serperior has a shallow movepool, we've been shown before that shallow but effective movepools can easily keep a Pokemon relevant. Look at Keldeo. It's been a massive threat since its introduction in the fifth generation--with a movepool consisting of 6-7 notable moves.
I think Serperior is another example of this. Leaf Storm and Dragon Pulse are a staple of offensive variants. From there, a player can choose either Hidden Power [Ground] to beat Heatran or Hidden Power [Fire] to beat Scizor and Ferrothorn, depending on what his or her team has more trouble with. Glare is awesome for the surprise factor, and can be taken as a fourth move if a player so desires, or you could go with Substitute to get an even better matchup against defensively-natured teams.
I see discussion about Knock Off on Serperior, and while I haven't tried it, I could see it being decent as well. If you're running HP Fire and your opponent has a Heatran, you could at least Knock Off the Leftovers on the switch-in, which Heatran would not appreciate one bit.
Overall, I think the depth of the movepool, while not helpful, doesn't stop Serperior from being effective. Like Keldeo, it can run quite a few different sets, even with its limited move options, which makes it a very interesting 'mon to use and face.