Looking at wp codex it says:
The register_activation_hook function registers a plugin function to be run when the plugin is activated.
I am finding this to be not entirely correct, for example i can activate a plugin that adds an extra field to WP registration form (using the add extra field example in codex) which does not require the register_activation_hook function (nor the register_deactivation_hook).
What codex does not tell you is when and when not to use the register_activation_hook, now i am going to guess that the hook is only required when including files for install or creating db tables on plugin activation, i would be grateful if someone who has more knowledge of WP and the correct use of the register/deregister hooks could clarify its correct usage.
regards