Peter,
I'm having fun but I'm also serious about the futility of that line. It is in fact a distraction that misleads the unwary, as you demonstrated when you wrote, "I still think it would be more difficult to set up without the lines printed on the armtube. That tapered tube would be tough to set parallel to the record surface".
Who cares? Do we play records with an armtube? Adjust what matters. Ignore the rest.
We play records with a stylus, so as you noted the ideal method for visually estimating SRA is to set the stylus with a magnifier. That's quite the bother however and as fine tuning must follow by ear I agree it's rarely worth the fuss.
The next closest thing to adjusting the stylus is to adjust the cartridge body. This is actually easier than levelling an armtube and arguably more effective. Just make the cartridge level to the record surface (or tail-up/down if that's what a particular cartridge prefers). Where the armtube ends up is irrelevant.
Who am I to go against the recommendations of the arm manufacturer?
You're Peter, I'm Doug, both thinking audiophiles who trust their own judgement and learn thereby. :)
With regard to pertinance, as the SME's line is pointless at best or a distraction at worst it is technically offensive, which makes it aesthetically offensive in a technical device. So I agree with you - the arm would look better without it!
Doug