Should I install using PostGIS extension? is an FAQ that comes up quite a bit in PostGIS circles and unfortunately askers get mixed answers.
In How to upgrade your database to PostGIS 2.0
we covered WHY you should use extensions. In this article I'll use my Lincoln-Douglas debate skills to argue why you shouldn't.
Keep in mind that I put a great deal of effort in fitting PostGIS extensions
into the existing PostGIS build structure and I eat my own dogfood, so I might be a little biased and a poor defender of the counter argument.