Riposte for Jedi Guardians provides a bonus of 5% to defense and its up all the time cause it always procs. So technicaly you could have your defense at about 16+ and be alright.
I do think that they take that into account when they set the levels you should aim for. To achieve those numbers, you often need end game gear and they recommend you have this build:
Meaning that they will count on you using all your abilities to the fullest, including riposte and 5% it gives.