I'm actually not arguing it from a office app perspective. I rarely use Open Office, and that's not where I first noticed it. I noticed it looking at a page in Firefox.
Whether preserving layouts or having good looking fonts in web pages is more important is just a matter of opinion. I think it's not something worth trying to get to some consensus about since its just the two of us (so far) on this bug, and it's a lose-lose situation anyways.
But as if you say, in the default config, the goal of 30-metric-aliases.conf isn't realized anyways, then I would say then that having those aliases is not worth the cost of every web page specifying arial looking like crap.
Again, as I tried to mention before.. this also has to do with what hinting and subpixel rendering settings the users uses. Nimbus Sans actually can look OK if you use 'no' or 'slight' hinting with subpixel rendering enabled. The problem is that the default ubuntu setting is 'medium' hinting with 'grayscale' antialiasing, which makes nimbus sans look bad compared to the other fonts.