For almost every possible application, OS X does not need an installer. If your application does not require special privileges to run, then please—do not put the application into a package. Here, too, as described for third-party installers above, a package tells the user that you are going to do something potentially very nasty to their machine, and this is the wrong message.
Like installers, packages usually force the user to put the software where the software maker wants it. This is just rude. As a user, I should be able to run your application from anywhere I please. Maybe I like having applications on my Desktop so that I have instant access to them. Maybe I want to put applications on a different partition to make OS updates easier. You do not know my logic, and your application should not care.
Smile On My Mac is one very obvious offender in this area. Their applications are great but have absolutely no need for a package to install them. Is a package really necessary to install an application that prints mailing labels? No; absolutely not.