The issue is that sometimes there are hidden console windows. I know that Chrome extensions for LastPass and IETab create hidden console windows. So I wouldn't want a hidden window to automatically attach when it's the only unattached console.
Another example is where you have TCMD (non-admin) and a CMD console (as admin). You might want to attach and try, but cannot. As Scott says, you would not want it to attach something unintended. I agree with Scott in this case to have a confirm before attach.