My son made rootbeer all the time. He'd do either a 2L bottle, or a bunch of 16 ounce plastic bottles. it was really good- just sugar, the rootbeer extract, and the malto-dextrine, and of course yeast. One time he must have mis-read the amount of MD, because he made bottled rootbeer Jell-O. It sure had great body, though- like Jigglers!
Seriously, that IS what gives the root beer the body and mouthfeel. In the correct amount, you should really have great root beer!