It is best practice to have well-thought-out URLs that do not have extensions. The reasons include:
- If your site’s back-end technology changes, all those URLs will either break, or need redirected, or be specially handled with have extra cruft forever.
- It takes longer to type and is harder to remember.
- It makes your technology obvious instead of receding into the background.
- It limits your ability to make awesome URLs, because now your URLs are dictated by how you name and organize your server-side scripts. (URLs are a user interface, just like GUIs, though they don’t get as much attention! http://alanhogan.com/url-as-ui)
- There is no benefit to outweigh the above negatives.
These are excellent points. A few of them I would have a small bit of disagreement over the perceived negativity, but they are too small to bother with. I would say it's more like there is not enough benefit to outweigh the perceived, possible, and/or actual negatives. But that seems a silly statement to make in of itself.
I think it revolves what your website actually is and how deeply invested you are in your URLs.
