I get what you are saying, most storage facilities, water mains, pumps, meters, plumping, etc. are all metal of some sort or another. -I'm not arguing that at all.
I also want to acknowledge that people can get overly concerned about the effects of metal (and admittedly, I may do this as well).
Just trying to explain my thought process/concerns a bit. I think the water travelling through metal pipes is going to be somewhat stable- there are limited chemical reactions going on, so the water will hit the saturation point of whatever metal ion/compound, and then likely stay at that level. Before it's added to the tank, it's treated with a conditioner, many of which have chelating compounds that can bind/detoxify those metal ions already present in the water.
If it's in the tank, then there are constantly reactions and biological/chemical processes going on, and as those ions get bound, the water will be able to dissolve more of them if a source is present in the tank environment.
Granted, there is a good chance that as long as semi-frequent water changes are happening, this may be a non-issue.
On a side note, have you ever turned on a faucet that hasn't been used in years? Or seen a sprinkler system go off? :P that's some of the foulest stuff I've ever observed.