I don't think it needs to be an either/or proposition. I really do appreciate the fact that you are endorsing proper technique and using one's ears to help produce good intonation but I think in your post you go a bit too far. One shouldn't assume that just because one uses visual references that:
1. They are thereby abandoning their ears using visual cues alone.
2. They are beginners.
3. They are not also using correct posture or position (not sure what you mean there) or their ears.
When you say "All lines do is provide someone with untrained hands...." I have to disagree. Highly trained players use these things. It is one more tool to help you. It doesn't mean that you abandon all else if you have lines on your fretless.
I didn't assume anything. My point was to counter the idea proposed that lines inevitably lead to improved intonation and that, by extension, unlined players cannot achieve or exceed the same security of untonation as lined players. This is simply not true. Which actually infers that it anything but either/or. I only mentioned inexperienced fretless players 1) in the context of perhaps getting a push-start from lines which could get them appearing to play in tune faster than they perhaps otherwise might have done, and 2) they are the ones most likely to drink the instant-results-kool-aid. But like all 'accelerator' methods the returns diminish over time - provided good techinique is developed the usefulness declines. Of course development of good technique is solely the responsibility of the player and for some it never happens. The risk with lines is that you come to rely on them and either never develop good technique or never develop full trust in your hands. Which is fine, and maybe the better option for the occasional fretless player whatever their other experience.
By 'position' I was referring to reliable shifting whilst maintaining hand posture. In other words the hand and fingers working as a unit, with the arm moving just the hand, and the fingers dropping into the right place without wiggling, rolling or twisting. Just straight down in exactly the right spot.
Goodness, OF COURSE you need to know what in tune sounds like. .
.... Where did you ever get the idea that the speed and weight of the bow alter the pitch? The length of the string is what determines the pitch and this is altered by the left hand. ...You can only alter the note very slightly by altering speed and weight of the bow and that is VERY slightly and temporarily to the point of it being negligible.
... But don't take my word for it, try this experiment: Take your upright bass (an EUB is fine as well) and your French or German bow and play an Eb on your A string. Now without moving the fingers of your left hand and using only the speed and weight of the draw of your bow (in either direction) produce a long tone F, D, or even an E. The results speak for themselves. Even producing an altered long tone Eb - sharp or flat - would work. Again the results speak for themselves.
...
My point wasn't about using the bow to radically adjust pitch (or even play a different note - really? How did that come through in my post?) but about being mindful of how different types of movement of the right arm may induce movement in the left hand relative to the instrument which can in turn impact intonation, and how good posture and active listening can help in this regard, whereas visual markings will not. The same can happen with electric bass - moving the right hand to a different playing position can change how the instrument hangs on the body which in turn changes the 'attitude' of the left hand relative to the fingerboard in a given position. Again, a little mindfulness and some good posture helps keep things in check. Visual markers do not as the viewing angles change all the time.
But whether any of this matters really depends on an individual's acceptance of what is good enough. For a lot of people, 5 cents either way is acceptable as 'good enough'. For me, as I've said in other threads before, 2 or 3 cents either way gets my ears burning and is not acceptable.