Uhmm..it's sort of becoming too much discussion over a simple variable. Why bring in Arch in discussing $DESTDIR? There's nothing "deep" about "Arch's way of building". It does not have to be "DESTDIR", some developers only define and call, say, "PREFIX", in the install rule and use it as a DESTDIR while the actual prefix (/usr or /usr/local) is determined in other ways.

It's trivial to work around it when a developer does not have a DESTDIR provision/equivalent, for instance: if you don't want to patch, just look at the install rule and follow suit in your script (whether Pacman PKGBUILD or Portage ebuild or Foobar goosebumps).

It isn't wise to push a distro's packaging shebang towards an upstream developer. But yes, DESTDIR is good practice and most software have it or something similar :) Nothing harmful.