There was some confusion over the rule's meaning. Some of the commenters indicated that since the rule requires that the trainee is appropriately rated, if the trainee was on the controls, he'd e able to log PIC under the "sole manipulator" clause. Others expressed concern about yet another PIC-logging rule that didn't even require "hands-on" flying time. Best I can figure from the explanatory material , like part of the extension of "supervised solo" it has a CRM basis. The trainee is being allowed to log PIC when "playing captain" for a multi-pilot crew even when not doing the flying.

Why the FAA found it necessary, I've not a clue. FWIW, this is what the final rule said about that:

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The rule was initially petitioned for by Saudi Aramco. Saudi Aramco wanted permission to allow new hires' training in their Bell 214 helicopter to eventually become PICs in the company and allow logging PIC flight time while under the supervision of more experienced and senior PICs. This rule does require pilots to hold at least a commercial pilot certificate and requires those performing supervising PIC duties must hold either a commercial pilot certificate or airline transport pilot certificate, and flight instructor certificate with the appropriate category and class of aircraft being flown, if a class rating is appropriate.
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If you're curious, the discussion of the rule and comments is in the August 21, 2009 Federal Register at pages 42514-42515.