that is a common argument. it is valid don't get me wrong.
my take here is
- it will stretch the main-game, so theres a cost
- side-stories can have a positive effect for the dev, theres a reward to finish a standalone project, and a reward to do something "different".
- that could increase motivation and morale, when u can do different things, take a break from the main-"big"-project.
- that "cost" of time, could now instead be "the survival" for a dev.
what Im trying to say is that, we have devs that abandon their work, for one reason or the other, and its not always after version 0.1, we have those that left after version 0.15.
I think its good to take a break from the main project and do something a bit different. and the side-stories are that.
also, making different things will give the dev other ideas, that could benefit the main game.
so, its not always a "cost", but an "investment", it all depend how u see it, your perspective.