pg_dump transaction's read-only mode

I'm looking at the following code in pg_dump.c

/*
* Start transaction-snapshot mode transaction to dump consistent data.
*/
ExecuteSqlStatement(fout, "BEGIN");
if (fout->remoteVersion >= 90100)
{
if (serializable_deferrable)
ExecuteSqlStatement(fout,
"SET TRANSACTION ISOLATION LEVEL "
"SERIALIZABLE, READ ONLY, DEFERRABLE");
else
ExecuteSqlStatement(fout,
"SET TRANSACTION ISOLATION LEVEL "
"REPEATABLE READ");
}
else
ExecuteSqlStatement(fout,
"SET TRANSACTION ISOLATION LEVEL SERIALIZABLE");

Is there a reason why we do not the RR transaction as READ ONLY above
? I understand that unlike in the case of SERIALIZABLE transaction, it
may not have any performance impact. But isn't it a good practice
anyways to guard against any unintended database modification while
taking a dump or a safe guard against any future optimizations for
read-only transactions ? More so because RR seems to the default for
pg_dump

Thanks,
Pavan