My HG skill fail me a bit, but apparently since Type Inference, somebody removed the call to attachArguments in GetElementIC::update. I guess this has been disabled because TI can optimize arguments access. But this might still be a win, because from what i remember this optimization doesn't work cross-functions.
Looked around a bit ('hg log file -p' for a complete history of changes to a file), and the arguments IC has never actually been enabled. Here is the original landing of bug 616744: https://hg.mozilla.org/mozilla-central/rev/da50621162f3 attachArguments is defined but is not called anywhere.
Brian activated the IC in Bug 703721.