The Google Account was already in use before Google+ was created. The difference was that you could have a mixture of services enabled for your account (I only use Gmail, I never use Picasa so I don't have it on my an account).

Now they seem to force you to always have the Google+ enabled even if you never use it. Including for older accounts. I guess this is what makes people angry. It's like being prompted to create Picasa albums every time you try to read your Gmail.