Hello first poster here,
this is less about mgear specifically and more general rigging workflow question. I am very much a beginner at this. As far as I have gathered, the mgear workflow should be like this: Shifter → ngSkinTools → RBF Manager → (Blend-)SHAPES.
I managed to understand how RBF works, but Blendshapes escape me completely. I tested SHAPES under the current version of mgear and it crashes with mgear if I use the weightDriver from SHAPES through the RBF manager (again I followed the tutorial too close, which is actually outdated now). Is there a way to add Blendshapes towards the RBF Manager without using SHAPES? (My 2nd question is how to practice on Blendshapes in the first place, because I struggle to see an endgoal for using them.)
In the lastes mGear version we have a variant of the weightDriver solver called mGearWeightDriver. Please use always this one with the RBF Manager.
Check the mGear Youtube channel, I did some videos about SHAPES and Corrective blendshapes in the past 
Cheers!
The end goal of corrective shapes is to “Art direct” the skin deformation.
If you don’t use Blendshapes, (I’d rather use the term “corrective shape”), your deformation will pretty much be the result of your skinning which can be limiting in some cases.
As an example, very often on rigs that don’t use corrective shapes and for facial specifically, you will be missing what is called the “naso labial fold”
Here is a quick demo showing how the naso labial is triggered when using corrective shapes.
and in that example from the Appolo rig by Ramon Arango, you even get a second crease after the naso labial.