The MPI Standard does allow for this kind of usage.
If there are more that a "handful" of ranks that need the same buffer, or if this communication patter will be repeated more than a "handful" of times...then creating a comm with the relevant ranks, and using MPI_Bcast would be preferable.
EDIT:
To clarify my own answer. The MPI 2.0 Standard specifically prohibited this kind of usage. The restriction was to accommodate Fortran. The MPI 2.1 or 2.2 Standard included a "clarification" that this re-use of a send buffer in multiple ISends was permissible. See Section 16.2.2 of the MPI 2.2 Standard for more.