D4DLCD_FillRect will only be used if it has been declared by the driver. As it would default to NULL anyway I do not see an issue with backwards compatibility.

A driver like the S1D13506 has bit-blit functions and requires this kind of feature to make updates fast as possible.

Most other drivers I have used offer a way of supporting drivers that support certain primitives, line, arc, circle, rect etc. Sometimes the driver may know a better way.