Maggotkin can reach 6 + 2D3 from turn 2 onward if their first turn is a bit lucky, if you add in some of the extra mechanics (e.g. the guy on the snail that can plant extra trees, or better yet his battalion) you can get far highter than 12, especially if you then spend your summoning points on more trees. See what I posted
here. It is possible to reach a point where you gain enough points per turn to summon an unclean one (addmittadly it takes till turn 5, but still). And that's with just the base-generation and summoning trees, there's no extra hero or battalion helping out yet.
What's more important though is that they generate points by doing what they're going to be doing anyway. Summoning those trees doesn't just generate more points it also gives control over the battlefield as those trees will make your guys faster while hurting your enemies plus they can be used as cover or even to flat out block a choke-point. Even without the generation of summoning points that's beneficial. Similarly a lord of plaques can generate points simply by scoring wounds on an enemy. It's a front-line hero, 'he's going to be punching people regardless.
In our case however we're stopping a slann from doing the one thing they're supposed to be doing. Which is counterintuitive to say the least.