In [this thread](https://boards.pbe.leagueoflegends.com/en/c/client-new-features-feedback/i7dOAcvy-new-runic-echoes-missing-the-ms), I wasn't quite sure how the new JG item would turn out. After playing quite a bit on this patch, I'm convinced that MS is a core stat needed for AP junglers to be viable without being overpowered through possibly just raw burst (which is also no fun to play against).
AP junglers generally don't scale very well and part of that is their opportunistic nature. They require setup and good positioning. They don't just go in and DPS someone like a Yi can. They're not faceroll and can't carry particularly well late game. While the reduced jg income/the team comp. still requires them to get a little tankiness, it's not comparable to something like Kayn or the countless other AD bruisers (who also have sustain on top of it). Kayn just hard outscales AP JG picks like Ekko, Fiddlesticks or Elise. He becomes more slippery than Ekko (which he uses to suffocate you by taking camps), has on-demand CC with W, destroys frontlines like a god and provides utility with Black Cleaver on top of it while also having a FAR superior clear speed all game as well as being more useful in the average teamfight. His extra tankiness and DPS also makes him better at objectives. He's probably not even OP but it just shows the sad state of AP champions in the jungle. How can any AP pick compare with a Graves, Xin, Udyr, Kayn, Yi or Shaco etc.? Not only are they worse at clearing, but they are also much worse and less reliable duelists (even more punishing in this forced meta), far worse late game and only situationally better early - mid game (low reliability again).
For their play pattern to work, MS is core. They need higher mobility to get their setups and land abilities/pressure the map while their abilities still matter more than a Graves auto. This lets them be more useful mid-game. They also need to be able to run from these strong AD champions who will no doubt destroy them in a duel. AP junglers are more about subtle kiting than something like a Xin and you want to be able to use your game knowledge, good prediction/aim etc. maximally in the early-mid game. You don't ever win a power farm war as an AP champion and unfortunately, scaling's not attractive either so you're forced into map plays. MS plays into that perfectly. More AP doesn't improve their play pattern while also being a bit frustrating to play against when snowballing. It feels less like an outplay and more like "oh, I just got one-shot by massive raw AP" rather than, "wow, she sneakily positioned here already for the cocoon."
Conclusion: Trade that 10 AP buff for 5% more MS etc. Apart from fitting their kits better, it's also much more fun to play AP junglers when you're zooming across the map and you feel less pressured by powerfarm junglers/late game junglers because you feel that there are options to take over the game before those champions get too out of hand. A little more AP doesn't change the massive scaling discrepancy or clear speed issues. You still can't beat them at that game.