Plasticizer is not needed, but it is highly recommended. Plasticizer reduces shrinkage, increases strength, increases flexibility, increases impact strength, decreases brittleness, and increases elasticity, all of which are good.
I used Ag14 as my sample without a plasticizer and Ag15 (a similar amount of agar, but not the same) as my sample with a plasticizer. Here are the results from some of my testing:
Ag14:
As you can see, it is all shriveled up and warped. It is extremely brittle and has some, but no much, strength.
