On SPV clients and Segwit An article released by Blockstream claims that after Segwit, SPV clients can verify that transactions have been accepted by the network without having to download the signatures/witness-data. 

I'm trying to understand why this was not possible pre-Segwit. Why was the signature data a part of what's sent to an SPV client, when it was of no use to them anyway?
Pre-segwit, the TXID was a hash over the scriptSig (signature data) and the outputs (value, locational data). SPV clients can not validate signature data by definition, so this is not useful information for them to have for anything but verifying the TXIDs position in the merkle tree. For segwit outputs the signatures can be stripped before being provided without altering the TXID, with massive data transfer savings.