transactions – why should the funding use the BIP 141 inputs?

In BOLT 02, we read that the financing TX should consume only separate entries.

The sender: when creating the financing transaction: SHOULD use only BIP141 entries (separate cookie).

What is the reason for this? If it was to prevent the malleability of tx, this should be a MUST. Also, I thought that consumed inputs did not matter much for this context.