Reasons not to use newcommand{{...}} (with double curly braces)

Question

Which are the reasons not to always use newcommand{stuffa}{{STUFF}} instead of newcommand{stuffb}{STUFF}? In which cases would the former be a bad idea?
The reason I am considering doing this is to make the commands work e.g. in superscripts without extra curly braces, i.e., 2^stuffa works while 2^stuffb doesn't (2^{stuffb} does). These commands will not only be used in superscripts, though, and I'm interested in a more general answer anyway. (If e.g. xparse provides a convenient solution to this problem, that would be interesting as well.)
I assume that this is a very basic question that has been answered many times, but I could not find an answer with a reasonable amount of research.
This question is related but does not answer my question.

David Carlisle · Answer

The definitions are completely different, only add the extra group if you want an extra group
newcommandzzz{bfseries}

would make zzz an alias for bfseries and zzz this  would make a bold this
    newcommandzzz{{bfseries}}

would do nothing useful as the font change would be in a group that ends immediately.
newcommandrecip{frac{1}}

would define a reciprocal function recip{4} would typeset 1/4.
newcommandrecip{{frac{1}}}

would do nothing useful as recip{4} would be {frac{1}}{4} and raise an error about a missing argument for frac.
The fact that having the extra group in the definition makes x^stuff seem to work is an accident of the implementation and should not be used. The documented LaTeX syntax is always to brace superscripts.
In general it will change the spacing,
newcommandzzz{+}

will make zzz act like +
newcommandzzzb{{+}}

will make zzzb act like {+} which isn't usually wanted, and gets no space compare
 $1  zzz 2  zzzb 3$

Reasons not to use newcommand{{...}} (with double curly braces)

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