Metonymy, synecdoche, metaphor

As always, I had to look up the two words to decide whether what I meant was metonymy or synecdoche.

The way I remember: metonymy -> material (same first initial), as opposed to general concepts covered by synecdoche.
OK team, once you know the right term, then what?
I learned these words once. They did not stick,
except that I recognize that they are a type of metaphor.

Does anyone actually use the distinctions? (other than to show off.) We could say that in
"All hands on deck"
that hand or hands is a metaphor for the entire man; and that in
"When the cat is away the mice will play"
cat can be a metaphor for boss.
Would anything be lost?
I could see a use if we were tracking an author through his authorship. We could note that he used 543 synecdoches and 376 metonymies in his first book, but the ratio steadily reversed; his last book had 259 synecdoches and 488 metonymies.

Any other good uses?
Richard Maurer To reply, remove half
Sunnyvale, California of a homonym of a synonym for also. (previous articles in Wheeze thread)