I'm currently designing an esoteric stack-based programming language for code golf (International Phonetic Esoteric Language/IPEL) that, in its current spec, has a stack of values that the user can modify.
However, after trying some simple challenges (and coming from a more procedural and OOP background), I've been considering adding a single register and its associated instructions (stack → reg, reg → stack, etc) for the user to use. (This was in the version -1 language spec, but I decided to axe it in favor of pure stack manipulation.)
What are the pros and cons of having a user-modifiable register in a stack-based language?