It's pretty obvious adwaita-icon-theme is the successor of gnome-icon-theme. There's no reason to make users having to install duplicate sets of icon themes when they have both gtk2 and gtk3.

When running in gnome, both gtk2 and gtk3 apps uses Cantarell fonts by default anyway, just like the other parts of gnome-shell. When not running in gnome, gkt3 apps uses ttf-dejavu in my system when there's no specific settings (since the day that settings.ini is no longer shipped). gtk2 does the same if no default font is assigned explicitly. IMHO there's no reason to have "Arch-specific" default font assignment gtkrc. Having gnome-theme-standard depends on cantarell-fonts should be sufficient.

P.S. To be honest another reason I suggested this is because Cantarell doesn't look good (blurry and "dislocated") without autohint. I know people claimed that with the introduction of Adobe CFF that shouldn't be necessary anymore, but personally I see no improvement (at least for Cantarell).