Are standard users still useful when Windows 8 has UAC?

UAC is not considered a security boundary. What this means is that there is only (relatively) weak protection preventing malicious software from "escaping" UAC and gaining administrator access. (In particular, Microsoft do not promise to fix issues that allow this to happen.)

Personally, I always use a standard user account on my home machine, except when I'm actually administering the computer.

When it comes to your personal computer(s), "best" practices tend to vary widely and are often ignored. Everyone uses their computer(s) differently at home and security is different as well. You should do what you feel is the best mix of security and convenience for you. What works for others, may be wrong for your situation.

Personally, all my users are administrator accounts on Windows, its just more convenient. If I were to have someone living with me and they had to use my computers, I would give them a standard account. I would also be more conscious to lock or log out of my computers.

And remember UAC prompts on a standard account, might be due to an application having admin privs, not the user.