I think the problem with Warlock is that he's hard to fit into lanes with current trends. He's really not a good trilane hero (lack of stun, lack of slow, no killing potential until lv 6, which will be slow in a trilane), both offensive and defensive, so you cant really run him as a standard support hero. He does pretty well solo mid - last time I saw him he went pretty much even with QoP in terms of cs at like 15 minutes - but still falls behind other mids; close to 0 rune control, lack of killing potential before 6, easy to gank compared to say, Puck or QoP, and just wont give the same game impact as a Magnus, Puck etc solo mid. I think I also saw him solo offlane once as well, but the ease with which you can gank him, as well as having no reliable method to get farm, make him a pretty terrible offlaner. His spells are great though, particularly Fatal Bonds and obviously his ult, but he is just difficult to fit into line ups these days, particularly when probably his best spot (mid) means your missing out on running something like Magnus, Puck, Batrider, Storm, QoP in the same position.
He does excel in 2v2 lanes though, hence his ridiculous performance in pubs.