These cranks position the chainrings further out than the classic cranks you might be accustomed to. The primary market is mountain bikes, secondarily trekking bikes.

For example, in the early '90s, I was accustomed to having the inner face of the granny ring somewhere near the plane of the BB shell's driveside face. Nowdays that's not the norm, not for Shimano's stuff anyway. If they did that, the tail of the FD would be hitting the tread on the rear tire on many bikes, especially in this era of 29er popularity. Hence the 50mm chainline.

If you want to fudge the positioning by one spacer, you should still have enough bite on the non-driveside. Myself, I'd use metal spacers that won't split if they're under a lot of torque, and install the cups quite tightly... you don't want issues when you're on tour.