I generally agree with you... that said, I can see the chainline and trim on my front derailleur even with my chainguard installed since it has a little bit of a cutout there. My mixte is a heck of a lot lighter and more nimble than a dutch bike for the mega hills I need to climb in Seattle. I still want to do so with a little style, hence the chainguard. I think if you ride indexed, you are less likely to pay attention to the status of your trim and chainline than if you have a little chainguard installed.
I have a Raleigh 3 speed DL-1, and I fully intend one of these days to fully enclose the chain like it was meant to be. My three speed, is still more of a toodling down the MUP kind of bike than a hill climbing get around town beast.
This is a funny subject for me, because I ride 3 of my bikes a lot.
1973 Raleigh Competition: Even with my iPod going, I'll hear a little something going wrong and I have to look down to see if I have the front or rear derailleur a little off. Soon as I see it, I know what to correct.
1994 Motiv Sonora MTB with Gore Ride-On cables: Mostly perfect, 24 speeds and I can see what gear I'm in by looking at the indicators. If something's wrong or feels wrong, maintenance is overdue, usually. Usually a new chain or cassette is needed.
2001 Titanium Spectrum with 2009 SRAM components: Just like my '73(even though it's 20 speeds instead of 10), always checking for what gear I'm in and if I need to trim something (even though I can only trim the front in the big ring).
I like all 3 bikes, but it seems like full circle. Hard-Easy-Hard.