I am an ASP.NET developer for years. In many companies and projects that I worked for there is constant need for re-usable components. Some companies were completely serious about it so based their business on it. When I'm doing software projects on my own management (as project manager or as freelancer) I encounter this subject again.
Realistic or not realistic, it is very favorable if we could use previously developed components in new projects. It would be very time saving specially in repetitive projects. Code re-use exists in different levels of ASP.NET. You can use Html helpers or user controls to share components between projects, you also can use services like logger or authentication in several projects. There is a type of component reuse in ASP.NET that is used by modules like ELMAH that is based on HTTP modules or middle wares. None of them are a component re-use that I need. What I need is a complete set of component re-use. I need all core and UI elements all together. For example in logger example, I need core logic and all needed UI all together in a manner that I can plug into a new application so other components of the application can communicate an integrate with it. I know there is a solution for ASP.NET that is called Area that approximately do what I need. It do its re-use in view (UI) well. I just copy files into its directory. But I it no designed a really separate component. It is forced to be aware of mother application's internal functionality specially on database design. Maybe it is the reason that ASP.NET MVC area is not very popular.
I've read a lot about Django that is re-use friendly by design. I see it is based on apps. Also I see that there is an app sharing web site for it. But never used it in a real project.
By thinking more and more on software re-use (in the context of web development) I realize that not every component re-use is suitable for the application. There is trade-off here. If you want to have a re-usable app then you have to develop it as generic as you can. That itself is causing complexity and creating bug and even consumes more time for development. When you start using a component among several projects you must carefully think of every change you made in the application. Each change must be confirmed as backward compatible as others are using your app. So maintenance would be hard. Apparently this is the reason many web development teams do not employ re-usable components a lot.
There is at least one situation that this model of software re-use makes sense. When you produce a re-usable app for a limited range of projects and limited period of time and when you are intended to use your app only in a family of project, that would better suites. Here it is good that Django applications are developed in this manner by default, whether you wan to re-use it or not.