where the studs are 16 inch OC use the one half inch, where the studs are 24 inch OC, especially on ceilings, use 5/8 inch. The reason for the thicker rock on the ceilings is to stop the bowing or bending where joists are 24 OC.
I do realize your initial question was NOT 5/8 inch but 3/8 inch.
But, 3/8 inch anywhere will bend and be too pliable when you lean against it.
Also 5/8 inch is always recommended on ceilings where accoustical texturing is used. Its a weight thing.