Example Magento1 install script with some explanation.

install-0.1.0.php

<?php
// I almost never use $installer = $this; instead I prefer to use the following

$installer = Mage::getResourceModel('catalog/setup', 'catalog_setup');

// This is just an example of instantiating the setup class in the script. Of
// course I choose the appropriate setup class and resource on a case by case basis.

// That way it is very visible what setup class is being used. It can also be
// switched within a single setup script, for example to add attributes to
// catalog_product and customer entity types.
// Also, PHPStorm + Magicento give the IDE knowledge what the returned type is,
// even without a phpdoc type hint.

$installer->startSetup();
$entity = array(
    'entity_model' => 'example/foo',
    'table'        => 'example/foo',
);
$installer->addEntityType('foo', $entity);
...
$installer->endSetup();

// Instantiating the setup class in the script resolves the issue that
// the type is defined or might be invalid after configuration changes.
// For that reason I see no additional benefit in adding an IIFE for
// for type safety.
// Regarding the scoping of variables in the setup script, using a custom
// setup class with custom methods is PHP 5.2 compatible and fits the OOP
// architecture of Magento more closely.
// If lots of data is needed, that can still be supplied to the setup class
// methods as arguments (i.e. the usual Magento configuration arrays), or
// a specific, one-time custom setup class could be used.

// (Personally I also find the PHP IIEF syntax with call_user_func ikky :))

// Regarding a method not warrinting a class of its own, usually I try to
// keep classes as small as possible. Single method classes are find, but
// actually are very rarely needed.

I share your feeling regarding wanting to "wrap" code somehow, be it a function or a class.
My comfort is that it is wrapped inside the method call in the setup class. Not much comfort though.
When using a framework I generally try to stick with it, trying to use it in the way it is supposed to be used.
For one I hope to benefit from the consistency myself, and of course I would like it if other developers have an easier time reading my produce. So thats my reasoning why I try to do things the way I do.
When it comes down to it, I think the IIFE are fine, except for support of ancient but officially supported PHP versions.

Regarding your question, without a <class> node in the config.xml the detault class Mage_Core_Model_Resource_Setup witll be used as the default setup class.
I'm not aware of any resources recommending that approach besides the official Magento U developer trainings that I give :)

Oh, and you are right in that I usually don't specify that <class> node in my config XML.