Why does structure matter?

[Mandryka]

cercare, in Italian, means search, to search, searching.  I am not sure how this translates into a musical application - but the text you quoted would otherwise seem to support Rubsam's approach to some degree.  Whether or not Rubsam's example is an exaggerated manner of playing Bach in the way indicated in the text quoted from the letter, we will never know.

Indeed. But I do like what he does more and more not less and less. It's true that I don't listen to it for more than half an hour at a time, but that's true of all of them if I'm not a concert.

[Crudblud]

I kind of appreciate that Rubsam Goldberg, it's better than hearing it played the same way every time. This is also why I like Vartolo in Bach and Froberger, he lets the notes "breathe" in an interesting way. But I also love Scott Ross, so I guess I'm just into the thing one way or another.

[aukhawk]

... IMO, there two defining aspects of music: 1) organization of sound and 2) an intention to create music, both accomplished by a human being(s).

Is there no music in birdsong?
The readily-identifiable descending cadence of the cuckoo has been appropriated by Beethoven, Delius and many other composers wishing to evoke the rural idyll.  Though why we get sentimental feelings over this arch-criminal among birds beats me. 
I like the blackbird better.  I can and do sit for an hour or more listening to a blackbird, given the opportunity.  It stimulates my mind in exactly the same way that keyboard music by Bach does***.  The blackbird never quite repeats itself.  It has a few stock riffs and trills that it favours (different individual birds favour different phrases), and permutates and adds unique transitional notes each time, so that my expectations are always confounded.  It is also usually responding to one or more other nearby blackbirds, imitating and augmenting their song.  So arguably it is just making conversation.  But to me it sounds intensely musical.

I agree with amw and others - structure may be important, possibly near-essential, to the composer, it may also interest the academic, the performer or anyone reading the score - but it isn't essential for the listener to appreciate it, except maybe in those limited musical forms which are predicated on structure, such as Classical sonata form - but they are a very small subset of music as a whole.

But even when listening to Haydn, as a listener I prefer the analogy of walking along a patterned carpet, which is unrolling in front of me and rolling up behind me.  Thus I am listening 'in the moment' but with some element of expectations being raised and met or confounded.  But that's just me, and I expect educated 18thC concert-goers were different.

I also think there's some confusion/crossover between 'structure' and 'organisation' in this thread.  To me, 'structure' operates on the macro level - for example the symmetry (I'll avoid the word 'arch') of Mahler's 7th.  Instrumental timbre, and simple cadences, are organisation.  Riffs and grounds may be either.  Bricks laid in Flemish bond are 'organised' - lots of them and you get a wall - 'structure'.
One structural analogy that is often used, that personally I find very unhelpful, is that of the arch.  I cannot relate the visual experience of an arch, stable and static as it is**, with anything that I hear in music.  You see both legs of an arch simultaneously.  In music with a symmetrical structure, the first leg is past and gone, the second is as yet unknown.  It's completely different.

** yes I know that the actual dynamics of an arch are anything but stable and static - but that's the visual impression.

*** actually, I have often experimented with combining them - by sitting in a location with lots of birdsong (my garden, or an Alpine meadow) - and playing music on open-backed headphones.  I usually find time and space to do this when on holiday.