Ok. Colour me dumb, but what exactly is the whole purpose behind this "poser debut"?!

You can buy the full program, use only the parts that you want to, and have the parts that you can't yet use, for when you feel up to speed enough where you want to learn more.

I mean one can play "dress up dolly" for only so long before you want to start sinking your teeth into some of the more substantial elements of the program... such as the material room and cloth room.

Will there be plugins available that people can buy to add to the "debut" product? Or will they have to fork over more money to buy upgrades?

Personally my opinion is that if you want to get into 3D, rather than spending money on something that will take you 25% to where you need to be, you are better off spending the money on something that will eventually get you 100% to where you want to go.

Much of the advertised art is post worked, and have elements in them that are done using more advanced elements of the program such as material room nodes, and lighting. Lighting is the key to a good image, but so is knowing placement.  No out of the box program will get you doing those things within minutes unless you already know how. 

"It is good to see ourselves as others see us. Try as we may, we are never
able to know ourselves fully as we are, especially the evil side of us.
This we can do only if we are not angry with our critics but will take in good
heart whatever they might have to say." - Ghandi