In which cases may I use an installer?
The first question you must ask yourself is: "Do I need an installer to install my applications or documents?"
For instance, if you're distributing an application without any other files to install (Frameworks, StartupItems, Drivers, etc.), you don't need to use an installer in 99.99% of cases. The 0.01% left is made of unusual cases like for instance if you want your application to be installed in the Applications folder of the root home directory (but this is just an example). For the other 99.99%, just distribute your application inside a Disk Image and inform the user in the Read Me file he/she just needs to move the file into the Applications folder to install it.
If you're distributing a solution that needs to put a Driver in /System/library/Extensions, an application in /Applications, a Framework in /Library/Frameworks, then you seriously need an installer.