in the debate of ArP vs AD vs AS
depends on what you aim to gain from your runes
a) stats that cant/are harder to get in game
b) a good starting bonus
if you choose A then ArP is the way to go since there is 1 (2 if you buy brutalizer again) items that offer flat armor pen (also for this scenario MS quints also count)
if you choose B then AS or AD with some armor pen if you dont go 21 in offence so you can farm easily
Now about the original question which champs is AS good for, most junglers can benefit a bit more from AS like xin zhao who will proc his passive more, trynda who will get more fury or nocturne who will proc his passive as well and if you start with vamp scepter you ll notice that the extra 15% attack speed helps a lot more in lifestealling than 10 AD.
Keep in mind that the new jungle is quite easy to clear so there is no rune set up (given that you get some armor from your runes and some offensive stat) that will make you get crippled in the jungle , you might as well watch stonewall008 in youtube to see his rune setups and you ll notice that many times he says 'choice red/blue'.
junglers that don't really need AS runes lee sin, amumu , maokai (debatable), jarvan IV, or most junglers that pick cloth +5 hp pots.
Personally i usually use AS red and ArP quints for jungling.