Rules and referential integrity

I seem to have found a way to use the rule system for referential
integrity checks.

The manual says (in the "Rules vs Triggers part)
"It is possible, to place a qualified rule that rewrites a query to
NOTHING if the value of a column does not appear in another table. But
then the data is silently thrown away and that's not a good idea. If
checks for valid values are required, and in the case of an invalid
value an error message should be generated, it must be done by a
trigger for now."

Now: why not define a function (in pltcl; could be something else?)

create function err(bpchar) returns bpchar
as 'error $1' language 'pltcl'

Then, if the value of a column does not appear in another table, you
could rewrite the query to (for instance)
select err('your message')

This will raise a PGRES_FATAL_ERROR , with error message
ERROR: pltcl: your message

So: it is possible. Why it is better (or worse) than a trigger, I do
not know. Views on that?