Grouped answer (french guy here, be indulgent ^^) :
Cross platform compilation is a requirement for the most important of makepkg's users, namely our package maintainers...
I think TU know how to configure makepkg for theirs binary packages to be portables.
user has an ageing AMD FX system and wants to replace that with an intel core i7 system. they don't feel like re-installing, so just transfer the harddrive to the intel system. if they used -march=native everything they build on the AMD FX system will need to be rebuild on the core i7 .
Well, that's a good point, I didn't thought about that ^^ But it's only about rebuilding aur packages, not the binary ones.
I believe when the decision was made it was simply based on the fact that being able to share is worth more for the community than local optimization.
PKGBUILDs and packages from repositories have to be portable, but it's not a requirement for self-build AUR packages. Who shares binary packages from AUR ?
Meanwhile you didn't make clear why don't you side with OP after justifying his point?
Sorry, I don't understand your sentence. What's "OP" ?
anyone rebuilding some set of packages with ABS to eek out a bit of extra performance is aware that the default compiler flags need to be changed
I didn't talk about that, all my point is about AUR building.
I'm sure there are users who build packages without devtools and then expect it to be portable
I think they should read a bit of doc before cross-compiling. I'm aware that it doesn't make a great difference for most packages, for only this portability downside so why private ourselves ?