They do not hate tray icons but tray icons are typically ugly and never match a design theme. There are so many iconsets it make sense that people dont make icons to match them all.
GNOME 3.14 and older just decided to hide them to make the GNOME design much better, which it did make them much better at the cost of functionality.
GNOME 3.16 on the other hand has actually improved the functionality while at the same time still keeping the nice design even with ugly tray icons. =
Screenshot of the new GNOME 3.16 Icon Tray Panel
The new tray icon panel is actually nice looking, until you open it of course but it easily hides away which is nice. The problem with it is that you can't control the location or anything else of it. You either use it as is or you remove it entirely.
Top Icons Extension by "ag" has solved this problem.
https://extensions.gnome.org/extension/495/topicons/
With Top Icons you can replace the bottom left icon tray panel with putting the icons back in the top right of the main panel. This extension also does the same thing with GNOME 3.14 or older.
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So conclusiong, GNOME doesn't hate tray icons but they are always ugly and never match anything. They chose to hide them because of this but now are trying to compensate by adding new functionality for them.