What's the driver needed for?

The driver is needed for:

Do I need to install the driver when I only want to use the AJAX Remote?

No. The driver is not needed at all by the AJAX Remote.

Can I use Remote Buddy without the driver installed?

Yes. However, some features and drivers will not be available in that case. You can install, testdrive and uninstall the driver at any time using the "Setup Wizard" provided in Remote Buddy. You can open it by clicking on Remote Buddy's icon in the menu bar of your system and subsequently selecting the entry "Setup Wizard..".