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macnab69 - your succinctly put post on why VL's suit your needs, made me realise that it also expressed why I don't use them. But that in no way invalidates your use of them, which as far as I can determine is exactly what they were intended to be used for.
Simple example - my current VL is all my books about beetles, I add a new book about carabids, I open 'N' in the Tags category in the Tag Browser, but I can't drag the book to "Nocturnal" because this is my first book about nocturnal beetles, so I have to...
If there's a way to avert that behaviour then maybe someone will tell me about it.
Begs the question - would I use them if they only restricted what books one could see in the spreadsheet but not what categories one could see in the Tag Browser - I dunno.
If I had a choice I'd prefer to have support for controlled vocabularies, or Quickviews of entities outside the scope of what I'm currently looking at, or a multi user system, or direct linking of books, or support for multiple instances of the same file type in book folders, or Hyper Libraries than span multiple physical libraries...
I only use a subset of the the panoply of Calibre features, I suspect that's true of most of 3.5 million active users.
BR