This is an outgrowth of a question from @Josephine in a thread where I mentioned having played a few pieces from memory at a recent "piano gathering".
I'm going to present two philosophies, one of which I find more suitable to me at this stage in my life.
The first is argued by Josh Wright in the video linked below.
Josh has, in the past few years, gotten his PhD. He competed in the U.S. Chopin competition in 2015.
His world view for himself and for his students is at quite a different level from mine.
The video argues against hoarding pieces one has learned. Drop them. Learn new stuff. Keep advancing. You can always revive the old pieces.
I think this makes sense for a lot of people.
But certainly not for me, at age 78. (Although I'm ancient, I'm not retired. I still work about 25 hours a week, so my time is not unencumbered.)
I don't have talent or training. And rather than spend time with a teacher over the years patiently learning to do things correctly, I've spent time learning a number of pieces that I love - mostly by patience and not giving up. Most of them at a difficulty level that any sane teacher would no doubt consider inappropriate for my skill level. But so be it.
I'm not looking for a career in music.
I want to enjoy music.
At the piano gathering, my gift to the hostess was a copy of Charles Cooke's book "Playing the Piano for Pleasure" https://www.amazon.com/Playing-Piano-Pleasure-Improving-Discipline/dp/1616082305/
I'm going to quote a few lines from Mr. Cooke, with which I quite agree.
(From pages 102-103)
"I have a quarrel with most piano teachers.[... ]
"We study the piano in order to play it, do we not? [...] Can you explain to me, then, why piano teachers thoroughly teach fine compositions to their pupils and then complacently let these compositions slip through their pupils' fingers? Or, worse, partially teach fine compositions to their pupils, drop them, and go on to new work before the old work is done?
"Of all sad words of tongue or pen, the saddest (and most inexcusable) are these: 'I used to play it.' Next saddest, and next most inexcusable: 'I began it, but I didn't finish it.'"
There's more, but that's the gist of his outlook. His book is geared toward amateurs who want to be able to play, and he has a number of suggestions on how to practice, memorize, select repertoire, etc.
So yes, personally I hoard. To be honest, there are dozens of pieces I have let slip out of my hoarding grasp. But I find resurrecting one of them faster than learning something new from scratch (if it was something that I sufficiently loved so as to have memorized it at one time).
Cooke's recipe for retention is to spend part of one's practice time slowly playing over a piece one wishes to retain.
Josef Hofmann, in his book republished by Dover but available free here: https://ia601209.us.archive.org/13/items/pianoplayingwith00hofmuoft/pianoplayingwith00hofmuoft.pdf
presents similar advice.
[
Page 116: A
remedy against forgetting you will find
in refreshing your memory in regular
periods, playing your memorized pieces
over (carefully) every four or five days.
Other remedies I know not and I see no
necessity for them.
Page 121: I recommend playing for
people moderately and on the condition
that for every such "performance" of
a piece you play it afterward twice,
slowly and carefully, at home. This
will keep the piece intact and bring you
many other unexpected advantages.
page 123: Since he cannot play a piece very
often without having a number of errors,
rearrangements, slight changes creeping
into it, he must, in order to eliminate them
and to cleanse the piece, return from time
to time to slow practice in which he also
refrains almost entirely from expression.
When in the next public performance the
right tempo and expression are added
again they tend strongly to renew the freshness
of the piece in the player's mind.
]
Etc.
So again, I think it's a matter of approach and emphasis. I personally wouldn't want to spend hours practicing the piano, but not having anything I could just sit down and play.
At the moment, I'm splitting my time three ways.
Reviving a Brahms Intermezzo I played many years ago, trying to re-learn a Rachmaninoff transcription for which I learned the notes prior to my open-heart surgery but which I subsequently dropped when I lost some use of my left hand, and keeping alive pieces that I can currently play.
For the keeping-alive time, I often play without pedal. (I read an interview with Ashkenazy decades ago wherein he said that he generally practices without pedal, and devotes other practice time to just work on the pedal). I play very slowly, maybe with some expression or maybe more mechanically, but always with a sense of the rhythmic pulse. Seems to work for me.
This is Josh's alternative view: