I've had friends use the MySQL replication extensivly (admittedly a couple of years ago), and it works for some definitions of 'works'
there are many horror stories around about MySQL replication eating people's data. being 'built-in' doesn't eliminate that possibility.
there are many different ways to do replication and failover for databases, each has advantages and disadvantages. Postgres (through these external projects) has most of the range covered, the hit-standby,synchronous-write mode is missing, and that is what is going to be added in 8.5
it may be that once this gets in, some of the other solutions that need patches will ask to be added as well, but so far it's been a case of trying to to imply that one solution is better than all the others by including it and not the others
most of the different replication solutions that are available for postgres are significantly better than all of the others, for a specific set of requirements.