I came across the following General Order from Wellington's HQ dated 11 May 1813. On the surface it seems very logical. . . it would lighten the weight of what the soldier had to carry. However it seems very short-sighted given that the soldier would eventually needed the great coat.
Perhaps one needs to consider whether you use a greatcoat to protect yourself from cold or rain. I think these coats were a great burden already in May, there in Spain und Portugal.
I was under the impression they still had blankets, and also tents in the 1813 campaign, in 1815 it was similar.