Bill I see what you are saying and I agree with you that the foundation of a good, well designed stand is where you have to start. A well designed stand, with a good quality lathe bolted to it will be rock solid when you are turning most spindle work and bowls for say a store bought blank that is without flaw and already trued up. I agree with you that in this case, the extra weight is most likely not needed at all.
Now let's change the parameters a bit. A not so well designed lathe, in a shop where the amount of space really limits the kind of stand you can have.
I have seen a number of workshops much more cramped than mine, they seriously do not have the space for a nice, big, wide, splayed leg stand, they have to make it somewhat narrower, add to that, the lathe they have is not top drawer, not even middle drawer. In other words the not so good quality lathe they do have, even without anything attached to it, vibrates badly at some speeds. This kind of lathe is most likely to have belts and pulleys to give the speeds, to the speed cannot be tweaked to go just above or below the vibration. I think that you will agree that most every lathe on the market has some RPM where the frequencies are just right to cause a vibration, correct? Wit a VS lathe, you can just tweak the speed and the vibration goes away, with a lathe that uses belts and pulleys, that cannot be done.
I think you would be open to the possibility that in a case where the space is limited, and a wide stand cannot be built, and where the lathe is not the sweetest smooth running one out there, that a few well placed bags of sand will make the lathe turn a lot smoother.
Now, in my case, I knew that I wanted to turn bigger weird stuff, if you look at
this recent thread, you will see a badly out of balance piece I turned and the final results are really nice. If my lathe did not have 400 lbs of sand etc on the stand, it would have been a lot more difficult to do, as it was, keeping the speed somewhat low, I was able to get this one into shape without much drama.
You are right Bill, in perfect situation, a well designed stand is the good foundation that should be laid down for the lathe, but, the world is not perfect, not even close (for me anyways), so it is not always possible to make the best stand, so sometimes you have to help your less than perfect stand, and add some weight.
The other thing I want to mention, when I had the C-man tube bed lathe, the difference that was made in changing from the standard good quality v-belt to a linked belt was amazing. For the guys out there with a standard belt and pulley system on their lathes, I'd really suggest looking into a linked belt.
Oh, here is an unencumbered look at my lathe stand, to fit into the space I had, I needed to make the legs less splayed at the tailstock end...........
I do not think this sacrificed much in stability, but, in a perfect world, it would have been wider at the tailstock end.
Cheers!