If you hook it up to your Google and/or other social media accounts, you should care about security because a compromised phone can compromise your account, spam all your contacts, pwn your Gmail, etc.
Even if you don't use a Google account and install only selected apps, you need an up-to-date OS for web browsing, there are hostile ads even off sites like Reddit due to ad networks funneling bad stuff through. Also there are similarly unsafe ad network content embedded in otherwise reputed apps.
