Well, if by "sequence" you meant "table in which you store a deliberately incremented value", then yes, with all the points you mentioned. The problem with that is, as you mentioned, all the ridiculous amounts of contention you get (and setting yourself up for big problems with deadlocking as a result). Of course, when you say "sequence" in the context of auto-increment tables, specifically in an oracle-specific context, it's pretty fair to assume that "sequence" means "oracle sequence", so I don't feel so bad for misinterpretting :-)
In one of my other replies in this thread I mentioned how you could do that etc, etc. The "right way" would be, in fact, to implement your own sequence by way of autonomous subtransactions (another oracle-specific feature... not to say that no other RDBMSs support them, but certainly not all RDBMSs support them).
Not an editor command: Wq