When you see it you may think I'm a troll, but this was how I've played him as a support main, I have had a few games as top or jg Jarven but I keep him down in support cause evern though he may not one shot the squishy and leave but never leave the fight and be the target in the team fight, First off Jarven at level 18 no items can offer you 26% armor penetration for 3 seconds and a 25% attack speed buff for 8 seconds! Which is great for pushing towers, and being a way for you to jump around the map WHILE ROOTED! he has great synergy with some of the AD carry's such as Miss Fortune and Lucian, Lucian for early game and MF for the ult combo wombo
Lucian is a great pick because when you both hit level two you both can dash into the team and hit them when you just shredded their armor by 10% and gave Lucian 15% attack speed buff. Also the Armor Penetration last for three seconds on hit targets while the Attack Speed buff last for eight seconds in a big radius around the lance