The Full Defense action is, mechanically, somewhat unreliable compared to the Defend action: Defend gives you a straight +2 Parry, while Full Defense lets you roll Fighting+2 to set your Parry. Since that's statistically identical to 2 + half Fighting for Extras, and only slightly statistically better if you've got a Wild Die, Full Defense doesn't seem very… full.
However, I just re-read the opening sentence of Full Defense (SW Deluxe Explorer's Edition, p. 82):
In addition to the usual Defend option, a character can go for a full defensive action.
I initially read that as just a fluffy introduction to the Full Defense action, saying that it's just another option for someone who wants to go on the defense.
Does it actually mean, though, that Full Defense is not merely an alternative to Defend, but is actually done simultaneously (the other way to read "in addition to")? That would make a Full Defense more effective, since you'd be guaranteed at least a +2 Parry, and more if you rolled well.