Get out your trusty 41c and execute ISG X. Is the old X saved in LastX? No. The same is true on the 34S for all six increment and decrement operations. INC and DEC are there because I've seen way too many cases of "ISG nn NOP" in programs. i.e. they are very useful short cuts. In this way INC and DEC are completely consistent with their kindred commands.
Not setting LastX for these operations has actually been a bonus more times than not. There have been a number of rather clever pieces of code written that rely on this behaviour. As an aside: I've contemplated introducing fast arithmetic operations that allow you to add, subtract, multiply or divide X and a constant argument. These would most likely not save LastX either.
Finally, as Marcus mentioned, I'm very unlikely to introduce LastX saving here, checking for X specially would require extra code which we kind of don't have space for. What about ISG, DSE, X<> or even CLx ? They change X but nobody has ever called for them to set LastX that I'm aware of? Must they too be modified to save LastX even though it breaks backward compatibility with everything?
Sorry, but I think it is consistent the way it is and that it is more useful not setting LastX. I'm going to be difficult to convince otherwise I suspect.
- Pauli