Reasons for a Magento Database Becoming Huge

Those tables remember the pages visited by your customers. They also have a connection with the compared products. If you truncate them some customers might lose the list of compared products. You can truncate them, but it could block the access to the database. I recommend putting the website offline while you do it.
Whenever you decide to truncate them back-up first just to make sure.

The <[modulename]_write> and <[modulename]_read> db resources are no longer required since Magento 1.4. Configuring them does no harm however, the core/resource class automatically falls back to the default_* set of db resources.
I guess most people still put them there because they follow old blog posts. The only valid reason would be if indeed a custom db connection would be configured, different from the default one, or to have your module support ancient Magento instances.

The <[modulename]_setup> resource is for using setup scripts with your module. The <class> node defines the PHP class that will execute the setup scripts found in sql/ or data/. If the <class> node is omitted, the default setup class Mage_Core_Model_Resource_Setup is used.