[addaon]

> typically you can just write the rest of the code with the roles of the values swapped

Today, yes; most modern instruction sets are pretty orthogonal, and you can use a value in any register symmetrically -- although even today, division instructions (if they exist!) are among the most likely to violate that expectation, along with instructions that work with the stack pointer. But in the XOR heyday, this was less true -- instruction sets were less orthogonal, and registers were more scarced. It's not unreasonable for an OS scheduler tick to do some work to figure out the newly-scheduled task's stack pointer in one register, and need to swap it into an SPR or similar so that the return from interrupt returns to the new location, for example; and this is the exact type of place where the XOR trick occasionally has value.