You are still vulnerable to:
- Cryptolocker-style viruses running on any other computers in your
network (including on any guest machines you might let on to your home network)
- Malicious or careless users on your own network
- Access by someone who gains access to your wireless network (WPS attack or weak/leaked wireless key)
- Vulnerabilities in the router exposing your information to the wider internet
That list is non-exhaustive, but covers some likely scenarios. Your data may also be vulnerable to hardware failure, power outages, etc but I've assumed you specifically mean security-related vulnerabilities.
In the end, it's up to you to trade off the value of the data against how much effort you spend defending it. There's little cost to setting up user accounts and passwords, and it gives you another layer of security against some of these vulnerabilities, so seems a reasonable precaution.