+1 from my side. I really need this ability for classes in Swift. Even if the class is 500 lines, it would still be useful.
Currently when segregating the functionality, we write conforming protocol methods in a separate extension. If the extension methods are a little big (e.g UITableViewDataSource), then I'd like to see this in a separate file where I declare the class as partial.
Another advantage we get from partial classes is that we can use private functions and properties defined in the initial declaration. Again taking the UITableView delegate as an example, if I declare the delegate methods in a separate file as an extension, then I wouldn't be able to use private functions/properties. I am forced to declare these without any modifiers (internal by default). Otherwise, I have to define all the extensions and methods in the same source file. This doesn't make any sense.
For those who say that logic has to be segregated, in many cases it's almost impossible or we end up writing unnecessary extra code. Let's take a scenario. I have page in mobile app, that has a table view and a collection view with different sections. The page also has extension methods to pick an image, document, date pickers, camera.
In this case all of the delegate methods have to be defined in the same source file even though I define these in respective extensions in the same source, the class looks much bigger. The reason of not being able to use or call private functions and properties limits my ability put these extensions in separate files. I cannot blame the UX designer to split the UI into multiple pages.
To minimize the no. of lines of code in a single view controller, I need to write content views and define the delegate and properties and these result in additional code where I need to assign the right data and handle UI flows. Here I end up writing multiple content views and view controllers plus view models.
All we have to do here is to allow the developers to have an option to keep the class declaration with methods and properties in multiple sources. The code reviewers will take care of the scattered classes in different folders. For individual developers I presume they study a lot and definitely be/become aware of best practices in the language they use.
Even with all good features and guidelines, we always find bad developers writing/misusing a lot of language features.
Adding partial classes is really a good option for mindful developers.