Basically, all answers in one:
1. Questing defense will not give you extra hp levels, is is sometimes faster.
2. There are some vital quests for certain types of pures with defense need. Some of these, and their requirements give additional defense xp... (i.e. defender of varrok for curses and turmoil)
3. Questing defense gives quest points, which not only can make your account more valuable, but it makes it more varied and opens up alot of different rewards... (Lunars, Ancient curses, Hand Cannon, Etc.)
It is like questing attack for a 1 defense pure. you don't want to have 63 attack after doing the grand tree and tree gnome village, so you get around 55 attack, do the quests, and finish it off legit.