This is required for the existing P2SH workflow as instantiated by https://github.com/zcash/zcash/blob/1f1f7a385adc048154e7f25a3a0de76f3658ca09/qa/rpc-tests/fundrawtransaction.py#L415-L447 - the caller uses validateaddress to get the bare transparent pubkey corresponding to the address, and then uses that in the construction of the p2sh script that follows.