Suppose I spellcheck a document with the word "blurfl" in it.  At the
moment, ispell will allow me to respond "a" when it highlights the
word, to allow me to accept that word for the rest of this spelling
session, but not add it to my custom dictionary.

If the document also includes the word "blurflize", the suffix rules
will offer me the option of accepting this new word as blurfl+ize.
However, it suggests I use "i".  If I follow the suggestion, I would
end up with "blurflize" in my private dictionary, but not "blurfl".

Presumably the sensible course in this example is to suggest that the
user uses "a" to accept that for the rest of the session, if the stem
word "blrufl" is only acceptable because of "a".