[Lawrie]

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Your approach with different libraries per band only makes sense if the repertoire of each band is COMPLETELY DIFFERENT. As soon as there are songs you play in several bands I recommend keeping your sheets in a single library. 
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You could e.g. use "Source Type" for C version / Bb version / Eb version / Bass Clef, "Albums" for books and "Custom Group" for the instrument. You can filter for combinations of song properties that can be stored and recalled as "user defined filters". Believe me, switching collections and filters is faster and more convenient than dealing with many libraries.
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Hi itsme,
in effect, this is actually the case.  Just because a song name may be the same has zero bearing on whether the chart is the same.
E.G. a lead sheet for "Satin Doll" is only suitable for the small jazz ensemble I play Trom in, but one of the concert bands I play Tuba in uses a part written to about grade 3 in a different key, and another concert band I play Trom in uses a grade 5 part in probably yet another key, while one of the brass bands I play Tuba , or sometimes Trom in, is a transposing part at yet another grade, but the other brass band I play in wouldn't play Satin Doll at all.  Then there's the different big bands playing different levels , and different arrangements, and I play different chairs depending on circumstance.

Adding parameters like "Source Type" etc. that you mention are all, of course, useful and possible.  However they all require considerably more editing of every song than to simply have songs/charts stored in different directories per band, with different subdirectories per instrument/chair and using different libraries per band with different collections per instrument/chair.

Switching libraries often will only happen when I'm doing chart maintenance/installation at home, NEVER at a gig, unless, like today, I'm playing in 2 different bands at the same venue.

I happily concede that my approach may not appeal to others, but to me it is the most logical method of reducing the labour required to manage things.  Absolute minimum editing of song entries is one of my goals.  I am extremely grateful such facilities already exist in MS.  The ability to have multiple libraries ended up, in fact, being one of the deciding factors in my choice of software.

I also don't think I've asked for major changes.  Most of them relate to the already existing "Switch Library" page, which would never be seen by those that don't want to use multiple libraries.  None of them are critical to my use, but they are refinements to already existing features.

The only one that would be more visible, and which others might not commonly use, is quicker access to said "Switch Library" page.  Whether Mike chooses the overflow menu from the normal main page (I believe this is called the library page) or not is completely up to him.  I only suggested that location as it seemed most logical to me.  It wouldn't be seen unless one entered the overflow menu and it would be a single line entry in an already fairly limited list.

I also note that Mike has suggested having the entry appear conditionally on whether there are multiple libraries or not.  I'm perfectly fine with this, but it requires extra coding that otherwise wouldn't be necessary.

I don't see that I'm trying to get the wheel reinvented here, just improving what already exists.