I don't need all my ink to be waterproof. I do like to address envelopes and sign cheques with waterproof ink, so I do make sure I always have at least one bottle available.
It is a need for those people who use it in pen and wash sketching.
I am as baffled as you with the "unfortunately it's not waterproof so it won't make it into my collection" line that I read quite often.
I think I have reached a happy medium with FPN. I do visit - there is a lot of good info there. I do post occasionally, but mostly view. But here is where I draw the line - I do not visit their classifieds any more. I will not buy from FPN. So if they want to sell to me (hey, of course they do!), they better list elsewhere.
I have a pdf file called "SwanChart" that I think I downloaded from FPN - not sure. As I can't identify the author, I can't post it. But in relation to the postwar 4 digit numbering of Swans, here is what it says about the first digit -
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