This crazed, poo-flinging monkey is best taken when you're thinking of bringing Brick as well, or if you're taking Honour as your captain. This doesn't mean he has no use without these two, but his efficiency will suffer greatly.
Due to Go Ape!, Marbles gains Counter Charge when within 4" of his big bud Brick. Having a decently set up double counter charge bubble can cause your opponent major headaches. Normally, Brick can defend his fellow teammates, but not himself, but now Marbles and Brick can defend each other as well. Make sure to keep charge paths open, so your opponent only has sub-optimal positioning options. This can really set you up in leading the game clock and forcing some clock VP from your opponent.
On the counter charge, his on the second column of his Playbook is (in my opinion) the most important result, as you can push the opponent's model out of engagement range. If you're lucky, the only option the opponent has left is attacking Marbles, which will grant you Loved Creature's +1 TAC on all of your models.
Loved Creature also makes possible using Marbles as 'bait'. They have to delete him immediately, or suffer Honour's wrath in the following activation. So don't be too afraid with positioning him aggressively. At the very least, they sink lots of INF in taking him out, resulting in only 1 VP, and you receiving a Guild-wide TAC buff for the remainder of the turn.
When playing Honour, your main play for setting up take-outs is making sure Marbles is engaging your target. This instantly gives Honour a TAC of 8, along with +1 DMG on all Playbook results, due to the Assist trait.
Also, never forget Goad! I've ruined several full-stack captain activations by goading him, and then running away away from the scrum/goal! Make sure you always have an MP lying around for that Bonus Time!, as rolling 1 die most of the times is too risky.
When would I bring Marbles over Wrecker:
Whenever I pick Honour and think it's possible to do some take-outs.
Whenever I pick Brick and think I need a second defensive bubble.
Whenever my opponent has mostly 3+ DEF models, on which I can throw poo without much risk. 4+ is possible as well, but you better have that Bonus Time!
Good into: scrumming low DEF teams, teams that can't easily handle the double counter charge bubble (Blacksmiths, Brewers, Butchers, Farmers) // Risky into: matches where you won't pick either Honour or Brick
ROLANDOOOOO!!! Not picking Brick? Not picking Honour? This little ball of armored plating can bring enormous efficiency in controlling scrums or tracking down dodge-less wingers/strikers.
His Rollerball trait gives him a 8"/10" MOV, making sure he'll always be able to keep up with speedy strikers. If those strikers also don't have buyable dodges (Acrobatic, Where'd They Go?!, etc.), Follow Up makes sure he can follow those strikers for another 8". That's Not The Ball afterwards works like Horrific Odour in causing the opponent's model to have to pay another INF to Kick. Pro-Tip: when playing rather aggressively, this trait along with the Offside Trap Gameplan makes the striker having to pay 3 INF just to Kick!
Of course, ruining striker's lives isn't his only use. With Ramming Speed, which is free when activating Rollerball, you can totally change the opponent's positioning in scrums. Push their models deeper into your lines, push others out of engagement ranges; you can do quite a bit with 10" of movement! Remember that he also has a on his second column. Putting 2 INF on the mascot therefore isn't a waste of resources. He's also a champ at controlling Furious characters. Boar and vKat absolutely hate him!
Fun fact: Placing him in cover makes the opponent lose 4 (!) dice on their rolls! Sure, he has a DEF of only 2+, so it's possible for the enemy to hit all his dice, but most wingers/strikers only have 4/5 dice! This means that the only way they can pull the ball off of him, they are forced to charge in. Whenever they're afterwards engaged, they have to pay more INF to Pass or take a Shot.
Wrecker's only vice is 2" models. Ramming Speed only pushes 2", so Parting Blows are a thing. That's Not The Ball and Follow Up only work if he is engaging the opponent. So watch out with 2" strikers (Fishermen, Vitriol, Mist) or models with easy KD's (Farmer captains, Tapper and Hooper).
When would I bring Wrecker over Marbles:
When I'm not playing Brick, as the double counter charge bubble is just too clock efficient to leave behind.
When the opposing team doesn't have strikers with 2" reach and/or buyable dodges.
Good into: matches when not playing Brick (Alchemists, Engineers) // Risky into: 2" melee zone heavy teams, either with easy KD's or with 2" strikers (Blacksmiths, Brewers, Fishermen)
PS to @Mako: I'll also copy this into my first post, so you'll be able to find everything I write in that one post. The pings will just show that I've added more info.