Just to get this out of the way, UAC provides no real security at all unless it is raised from its default level to “Always notify” in the control panel. Make sure you have done that:
To answer your question, the only real security benefit to requiring a password is that it makes accidental approvals less likely.
UAC is not as foolproof as using a standard account. If everything worked perfectly then UAC alone would be fine, but there are occasionally UAC bypasses, and historically they usually only work if your account is in the Administrators group, so it is still best practice to use a standard account.
I definitely would not trust Microsoft to make software where everything worked perfectly 