The downside is even though you know the current directory due to window.location the API is written in the way it assumes you either want a default location like "desktop" or need the user to navigate to the directory before you can do operations on it (for security reasons) even from a local context. The user needs to select the directory once per fresh page load (if you've dynamically reloaded the current content then multiple saves need only prompt once so long as you save the handle).
The downside is even though you know the current directory due to window.location the API is written in the way it assumes you either want a default location like "desktop" or need the user to navigate to the directory before you can do operations on it (for security reasons) even from a local context. The user needs to select the directory once per fresh page load (if you've dynamically reloaded the current content then multiple saves need only prompt once so long as you save the handle).