The ability (though of course not the requirement) to declare head and body renders separately would allow you to return the header before your processing, regardless of wether that processing is more intensive in the controller or the main body view.
Deferring only works if having a stub of a request is sufficient to proceed to the next step in the execution path. Unless you are going to implement your own conditionals (which, admittedly, is doable in ruby,) then you are going to force the evaluation of the request as soon as you want to use it to make a decision.
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The ability (though of course not the requirement) to declare head and body renders separately would allow you to return the header before your processing, regardless of wether that processing is more intensive in the controller or the main body view.
Deferring only works if having a stub of a request is sufficient to proceed to the next step in the execution path. Unless you are going to implement your own conditionals (which, admittedly, is doable in ruby,) then you are going to force the evaluation of the request as soon as you want to use it to make a decision.