Doubts about oid

Hi,
I was reading about oid and default configuration of PostgreSQL. A couple
of doubts
1) Why is use of OIDS considered deprecated? Is there something else that
can be used in place of oids for user tables?
2) Is there a performance impact if we keep the default default_with_oids
to ON?
Googling, I came across this -
http://philmcrew.com/oid.html
But most of the links given at that page were broken and the page itself
did not provide a lot of information.
Thanks,
Jayadevan

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