The problem is, what is the sequence number you are talking about is exactly?
Or let me put it another way: if you do have a sequence number that provides a total ordering of your operation, then that is exactly what you should use as your timestamp. What Cassandra calls the timestamp, is exactly what you call seqID, it's the number Cassandra uses to decide the order of operation.
Except that in real life, provided you have more than one client talking to Cassandra, then providing a total ordering of operation is hard, and in fact not doable efficiently. So in practice, people use unix timestamp (with ntp) which provide a very good while cheap approximation of the real life order of operations.
But again, if you do know how to assign a more precise "timestamp", Cassandra let you use that: you can provid your own timestamp (using unix timestamp is just the default). The point being, unix timestamp is the better approximation we have in practice.