It is a line item in my manifesto that there are no synonyms in English. (J.L. Austin said there were no synonyms in English except "furze" and "gorse", but I'm not sure I believe him.) When writing, there is always something to recommend one word over another, even if only allusively (because one recalls author X and the other recalls work Y). Usually, there is a real connotative or evocative difference. (You've probably seen the Twain quote: "The difference between the almost right word & the right word is really a large matter — it's the difference between the lightning bug and the lightning.") These shades and echoes of meaning lend depth and power to writing, and when we lose the distinctions between words we lose depth and power. We lose tools.
(An aside: My wife complains when I use archaic or uncommon words, like "caprine" or "desman", because she feels they snag the reader's attention where there is no real significance to the snagging. I reply that those words are the right words, and "goatlike" and "muskrat" would not be the right words, and that is that. She is right, of course, that a word that is marked as unusual affects a reader differently than a word that is not, and I cannot wish that away, so I must be aware of it. But I have no compunction about sending a reader scurrying for a dictionary. I was encouraged to hear China Miéville express a similar sentiment at a reading. When I thanked him for it he inscribed this in my copy of Embassytown: "Words ought to be a little wild." —Keynes)
I was thinking earlier today about "cleanness" and "cleanliness", and how wonderful it is that they both exist. While their meanings overlap in some contexts, they differ in that cleanness is a state, and cleanliness is a habit. Something is either clean or it isn't; it can only be cleanly if someone has gone to the trouble of regularly cleaning it. An unbroken plain of snow in Antarctica may be a fine example of cleanness, but not cleanliness, unless someone has been tending it. Cleanliness comes with a history; it speaks to character. This is why cleanliness, not cleanness, is next to Godliness — it is often beyond our control whether we are clean or not, but we can always aim to be cleanly.
Of course, most readers call that idiom to mind when they hear the word "cleanliness", so it brings with it all the complicated baggage of religion — sanctimony, charity, piety, punishment. (Curiously, the first English appearance of the sentiment is in Francis Bacon's Advancement of Learning, and there it is cleanness that signals the divine: "Cleanness of body was ever deemed to proceed from a due reverence to God.") Cleanness doesn't have this baggage. It does have the awkwardness of a double "n", which might trip you up during a reading, but compared to cleanliness it is...well, clean.