Why is the privacy for using watch tower services increased with unpredictable commitment tx ids?
Actually, the BOLT has been designed with very strong privacy properties with regard to watchtowers. You can give all your commitment tx id first halves to one watchtower and it still doesn't learn anything about the second halves, others txs or anything else about the state of the channel (or which channel it is watching at all), as long as the full commitment tx doesn't show up on the blockchain. Once it does, it still cannot learn anything beyond this one commitment state. That's what this is done for and it is rather remarkable IMHO.