As @Patryk_F pointed out in another topic, you cannot upload app projects with extensions to the Gallery, this being due to security issues.
However, I believe this has a broader appeal. App Inventor is primarily here for educational purposes, to help young people get into coding, work on good logic, and to produce something they can be proud of. Educational establishments and their students make heavy use of the Gallery and most curricula go nowhere near extensions because they want their students to think, learn and work things out.
On the other side of the coin, app developers want to get things done, whether that be for altruistic or commercial purposes, and very often App Inventor just does not have the features or components in place for them to achieve their objectives, hence the requirements for extensions, to fill in the gaps.
I agree that over the last year or so there have been a rash of what may best be described as "test" extensions: in which partly or in full the extension blocks provided could be handled with the built-in blocks. Extension developers have got to start somewhere, and this is a good way for them to learn, and should be encouraged, as long as we see that this is a learning experience, and that these extensions are not really intended for general use.
There is a second category, where an extension developer has got their hands on a java library that can do something fancy, so they bang out an extension that does this fancy thing. Usually eye candy, and rarely of any great benefit. We find this in most programming languages, where the hard work of hand coding a method has been resolved by the production of a library.
Now we get into the L337
area, die hard coders versus lazy/newbie extension users, but really we should look back to App inventor's reason for being. Even in my advancing years (!) I find the challenge of working something out good for the brain and for the soul. I look to see what App Inventor can do by itself as much as possible, not because extensions are "bad", but for the satisfaction of working things out.
There are some great extensions out there, many of which really should by now have become built-in components. Long may extension developers reign.