Why is the obscured commitment number necessary in lightning commitment TX's?

Why is the obscured commitment number necessary in lightning commitment TX's?

The obscured commitment number for each commitment transaction is the lower 48 bits of:

SHA256(payment_basepoint from open_channel || payment_basepoint from accept_channel)

It is encoded in the lock time and sequence fields (24bits each) of the commitment transaction. I don't understand why it is necessary, the BOLT rfc reads:

This obscures the number of commitments made on the channel in the case of unilateral close, yet still provides a useful index for both nodes (who know the payment_basepoints) to quickly find a revoked commitment transaction.

Why doesn't the TXID of the commitment TX suffice as lookup index key?

http://bit.ly/2Wv2fhj

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