Just like tommy I have never used the services of a private guide in Athens, and group tours are certainly not my cup of tea.
Regarding the title of your post: "Yes, not, for what?" the "for what" part is in my opinion the most important.
What are you interested or passionate about?
Archaeology in general? What period of history? Social life? Mythology? Architecture? Art? Crafts?
There are places that may interest you and others not, even if they are listed as "must see". For those that interest you, do some research before visiting them and hire a private guide.
If you do not have any answer for the "for what" it may mean that you want to visit a place without even knowing why.
I have used private guide services in a few other places in Greece for specific reasons and have sometimes been disappointed because the guide's knowledge or explanations did not match my expectations on specific points even if the guide was very competent.