Immediate Constraints

The direct question is: what is the advantage of an immediate constraint?

My habit is to add constraints to my databases and my first lesson was to make them "deferrable".  But a recent fight
withpg_restore taught me that to do a pg_restore that is complex, you need to defer the constraints.  I cobbled a way
todo that as I do the pg_restore. 

But that raised a question of why not just make the constraints all "deferred" and simplify my pg_restore process.

Are immediate constraints more efficient? Does this relate to transaction isolation in that the data would be
consistentafter each statement and therefor give better stability when multiple transactions are running at the same
time?

My brain is asking this question because so far in my experience, the issues with constraints are solved by making them
deferred. If I made them immediate, would I just bump into a different set of issues whose solution would be to make
theconstraints immediate? 

Thank you,
Perry