The main reason the recess is in the back of all moldings is to "thin" the wood, reducing the amount of continuous rings in the wood that helps control cupping.
There is a fringe benefit of that relief in the back of trim, it helps get past some irregularities in the wall. I have had more problems with that relief cut than benefits when it come to base. The tapered edge of drywall at the floor coincides the relief cut in the back of the base, which then requires a shim to keep the base in plane with the wall. Not a terrible thing, but an extra step to do.
It is my belief that the relief cut or kerf as you have referred to it as, now a days has more purpose to go over irregularities, than to control cupping, since houses have better climate control than the old homes did when the technique was created. The trim is machined from wood that is kiln dried, which also helps in the reduction of wood movement.
In a nutshell, I would not hesitate to use 1X6 off the shelf, instead of 1X6 base. I used both 1X6 base and off the shelf 1X6 poplar for the base in my home. You cannot tell one from the other. I stained it too, looks pretty good if I do say so myself. The rest of the trim in my home is poplar to. FWIW.
The maple you speak of can take stain decently too if the wood is treated first with a wood conditioner.