...sometimes Liszt played magnificently, like no one else in the world, but other times intolerably, in a highly affected manner, dragging tempi and adding to the works of others, even to those of Chopin, Beethoven, Weber and Bach a lot of embellishments of his own that were often tasteless, worthless and meaningless.Taruskin in the Oxford History of Western Music attributes this to a change in musical taste that was taking place. He links Glinka's remarks to the fact that he spent a year in Berlin studying with Siegfried Dehn, an "apostle of the new 'classicism' ". In turn, he attributes to this new attitude the removal from the training of musicians of the ability to improvise. Liszt, for example, often ended his concerts with an extemporized fantasia, but pianists of later times no longer improvise (or only very few).
I'm sure that there is some truth to this, but doesn't Taruskin prove too much? Is it not entirely possible that sometimes Liszt did indeed add tasteless, worthless and meaningless embellishments to the music of the composers listed? Taruskin sees the 19th century as a time when the classical repertoire became fixed and 'inviolate' with a loss of spontaneity. While that is also largely true, I think we can see it from a different angle.
Personally, if I go to a concert or listen to a recording of Chopin, Beethoven or Bach, I do expect to hear a more or less true account of the score. I also expect to hear the performer making the score come alive with dynamics, phrasing, articulation--the thousand indefinable things that make for a great performance. The score is nothing but a set of instructions and a kind of graphic representation of a performance. The notes on the staff are both. Things like dynamics (ppp or fff) or articulations like accents or staccati, are instructions and exactly how and how much are indefinite. A score tells us a great deal, but also leaves a great deal up to interpretation.
What I don't expect to hear at a concert or on a recording is the performer's embellishments of Chopin, Beethoven or Bach. Each of these composers arrived at a final (or reasonably so) version of the piece which they sent to the publisher. Beethoven was known to complain that, despite all his care, the published string quartets contain hosts of misprints (which could include missing or incorrect notes, misplaced or incorrect phrasing, accents, accidentals and so on). The obvious ones have been corrected over the years, but some may still remain! Beethoven wanted performers to have a correct version of the score. This they could then bring alive for performance.
Improvisation and embellishment are of a different order, I feel certain. Chopin, Beethoven and Bach could all improvise, Bach and Beethoven with a skill that we can barely imagine. But their improvisations were of a different aesthetic quality and category to the pieces they took the trouble to write down clearly and carefully and send to a publisher. Those works, I'm also pretty certain, they did not intend to be subject to embellishments that might on occasion be "tasteless, worthless and meaningless."
Here as an example of Liszt's more extemporaneous style, is his Fantasia and Fugue on B.A.C.H. That is to say, using the notes that correspond to BACH in German nomenclature: Bb, A, C, B natural.
Now do you really want to hear him or anyone else putting some music by Bach through the same kind of treatment? Do you want to hear him Liszticizing something like this:
Where is the room for embellishment? In music this highly and beautifully organized, I doubt adding notes would improve it in the slightest. It would be like dressing up a fine roast chicken with catsup and marshmallows.