Checking signature only vs. P2PKH It is clear to me why it's potentially unsafe to only use a check on == in scriptPubKey. For one, not verifying a signature in the redeem script means that someone could impersonate the redeemer. From the Bitcoin developer guide: 


  This lets the pubkey script verify that Bob owns the private key which created the public key.
  
  Bob’s secp256k1 signature doesn’t just prove Bob controls his private key; it also makes the non-signature-script parts of his transaction tamper-proof so Bob can safely broadcast them over the peer-to-peer network.


Why would it be unsafe to only use a check on V(sigA, pkA, m={txprev_txid, txprev_outid, txprev_scriptPK, tx_scriptPubKey, tx_amount}_skA) == 1? It seems like this is what we're doing in multisig. I.e., have a ScriptSig that consists only of (a) signature(s) instead of sig and pk like in P2PKH.

Why don't we just check signatures instead of checking both pkhash and a signature?
In an input spending a P2PKH output, if you were to only check that the signature is a valid signature with the public key in the input, then an attacker could use their own public key and produce a valid signature and thus a valid transaction under this model. The public key is not retrieved from the output script nor is it provided by the output; rather it is provided in the input. Thus you must ensure that the provided public key is indeed the correct public key, hence the check that it's hash matches the hash provided in the output.

In multisig, the public keys are provided in the input, in addition to the signatures. They are in the redeemScript. To avoid the same issues (of the attacker providing their own public keys), the redeemScript is hashed and the hash is in the output script. There is a comparison there to ensure that the redeemScript is the correct script.

There is an output type where you do just provide a signature. This output type is a script where the public key is in the output so there is no need to provide the public key in the input. Only the signature is needed. This is known as a Pay to Pubkey (P2PK) output. These are not widely used because public keys are large and this makes it harder to give them out.