For me it doesn't matter what analogy is used, I have my own that I use - the concept of a "battery", or "spiritual battery" - I have activities that charge my battery and activities that drain them. That doesn't make them good or bad, they are what they are.
But the general concept, whether in poly or life in general is that we only have so many things that we can "cope" with before we become unable to give any more. For some folks, this is a daily thing, for others, like me, it can go over weeks.
If spoons or buckets work for you, then that's just fine by me - I won't mock you for using them. The concepts behind are what is important, and I can relate.
"Listen, or your tongue will make you deaf." - Native American Proverb