The problem with having 1000 SAFEcent and so on is that each would be a SD and if someone sent 100 SAFEcoin’s worth in SAFEcents then the transaction load on the network is 1000 times. The principle was that the load can handle SAFEcoin transactions easy enough because even transferring 1/10 of the potential worth is ~400 million transactions. But if you transfer 1/10 of the potential worth in safecxxx (1/1000000) then that is 400000 Billion transactions the network has to do. I realise that is an extreme case, but an attacker can simply send nanocents and only needs 1 safecoin’s worth being sent between two of their wallets repetitively to slow down the network.
Divisibility has then to be able to provide a lot of divisibility without multiplying the transaction load. Thus this idea of @anon40790172 allows for one extra transaction if there is a fraction of a coin to be transferred whereas cents/milli/micro/xxx means that if someone sends all the amount in the small amounts then the transaction load multiplies. Even sending .999999 would entail 999999 transactions potentially.