Thank You Victor!

I don't see the value of this approach. If you learn a tune in 12 keys, you're all set. Unless you are in the depths of non-western harmony

Now, I will play "devil's advocate".
In my years, I've played ton's of songs - covers, originals, my own - as a comping instrument, but somehow I've missed that very important bass guitar teaching "tool" - play it in all 12 keys.
Let's say, we would play a song from Chic or Earth Wind and Fire, or Mezzoforte, or Steve Wonder, or any "Jazz Standard", but I was so wrong and did not spend time playing and learning it in all 12(?) keys.
If I've played the original/self-concocted songs - I also did not learn it in all 12 keys.
 
Now, I will play "devil's advocate".
In my years, I've played ton's of songs - covers, originals, my own - as a comping instrument, but somehow I've missed that very important bass guitar teaching "tool" - play it in all 12 keys.
Let's say, we would play a song from Chic or Earth Wind and Fire, or Mezzoforte, or Steve Wonder, or any "Jazz Standard", but I was so wrong and did not spend time playing and learning it in all 12(?) keys.
If I've played the original/self-concocted songs - I also did not learn it in all 12 keys.

What if a singer you're working with decides it's a tad too high for her/him and demands you play a semitone lower? If you are in any sort of recording/live situation, you're screwed if you don't know your keys.
 
What if a singer you're working with decides it's a tad too high for her/him and demands you play a semitone lower? If you are in any sort of recording/live situation, you're screwed if you don't know your keys.

Let me see.
I'm in the recording studio and have only 10 seconds to react??? and immediately record Donna Lee?
Anyway, I know that ALL BIG and (low) name bass players always repeat that "mantra" play in all 12(?) keys, but...

If it's a sightreading deal in the studio than I don't have enough time to re-transcribe it in all 12 keys.
If it's a Chord chart deal in the studio - I will have a few seconds (or at least 1 minute) to check the chords/chord progression but I won't have enough time to play it in all 12 keys.

I'm kind of still trying to understand that expression "suddenly" you must.

About the singer's request "suddenly" to transpose a song. If it's on a regular basis - start writing the Nashville system way the chord changes, if it's a single event - than I would have a few seconds to think about it and prepare for that change.

In short.
I've played in all keys various songs in various music styles but I've had NO time to to play all those songs in all keys.
It's only my opinion.
 
There is no problem to move up or down a song by any steps - I just don't understand that overwhelming everywhere statement, "If you don't play it in all 12 keys, you don't know the song"
Most professional level players can transpose on sight. If you can do this, you don’t have to pre-practice in all 12 keys.

If you can’t transpose on sight, then learning a song in all keys is certainly a way to circumvent this skill. A really long way.