[CentOS] Yum curiosity: --obsoletes with update, useful or not?

Got a wild hair this A.M. and decided to inundate my two brain cells.
"Man yum" includes this:

update If  run  without  any  packages,  update will update every
       currently installed package. ...
       assumed to be a shell glob and any matches are then installed.

       If the --obsoletes flag is present yum will include package
       obsoletes in its calculations - this  makes it better for distro-
       version changes, for example: upgrading from somelinux 8.0 to
       somelinux 9.

Hmmm. Cell number 2 doesn't forget much. I recall several admonitions
that major upgrade should take the form of a new install. E.g. CentOS 3*
to CentOS 4*. The paragraph above *seems* to indicate that a major
upgrade might be doable? Moreover, it prompts the questions:

1) "What are the downsides, if any?"
2) "Has anybody tried it recently?"
3) "Have any results to report?"

No pressing need, just curious.

TIA
-- 
Bill
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