You control what you open stuff in with web intents. The site the document is stored on doesn't.
Say Google Drive supports web intents:
If you have a doc on Google Drive, and you install an intent for a Google competitor word processor in your Google drive, you can open it, edit it on that service, and save it back to Google drive.
Say Google Drive supports web intents:
If you have a doc on Google Drive, and you install an intent for a Google competitor word processor in your Google drive, you can open it, edit it on that service, and save it back to Google drive.