There are so many android apps out there that it is not feasible to expect all the developers to write them for Sailfish. At least with being able to sideload then the apps that might be useful will still be available. I am thinking of banking apps and more local apps where sometimes just being able to use it even if it isn't the best is still useful. Some people may never need to use any android apps. Some people will never need to use developer mode. But it's nice to have that option should one desire it.