I think the motivation is that the address adder already has the capability to increment/decrement registers by various constants: incrementing the PC, incremen

I think the motivation is that the address adder already has the capability to increment/decrement registers by various constants: incrementing the PC, incrementing the address for an unaligned word access, correcting the PC from the queue size, etc. If you updated the SI/DI registers through the ALU, you'd need a separate mechanism to get the constants into the ALU, as well as controlling the ALU to do this. But using the address adder gets this functionality almost for free.