Very true dan, good point.
Put an indicator in your bridgy spindle, on the table vise or whatever, push down on the back of the ram with only slight pressure, I was like wow, pretty much a limp noodle, a hard breath could just about make the needle move. We all have opinions, my opinion is that it is necessary to level all and any machine that has slide ways to preserve the accuracy and condition.
Cast iron moves extremely easy, why do you think they setup the castings on a 3 point when checking dimensions for scraping? If on 4 points it can and will sag under its own weight, deflection is quite amazing, very interesting, consider the same when the machine was built a certain way, then you go throwing a tweak into it..... do what you want with your own machines, but my machines are level, and checked on a monthly basis to ensure the foundation hasn't crept. My lathe has had to be releveled, on account of the concrete base I poured especially for the lathe to sit on, I don't know exactly why but it moved.
Fun fact, say you scrape your machine in my garage at 50 degrees, and she's perfect. Then it gets moved into a shop at 75 degrees, it will wear differently than it would if kept in the colder climate where it was dialed in. And yes I'm aware that in the home shop you may never notice in your lifetime but the daily use for 10 yrs will have an effect.
This is a good discussion