Do wormholes only allow FTL travel in “folded” spacetime?

Faster than light wormhole travel depicted in science fiction and in popular science articles seems to assume that rather than being flat (with dimples centered around large masses), spacetime is fairly lumpy in order to facilitate wormhole shortcuts.

Is there a plausible reason why spacetime would be conveniently "folded back on itself" as illustrated below?

• Based on observational evidence there isn't. – CuriousOne Jan 26 '16 at 17:30
• I am not proficient with wormholes enough to answer this, so I'd just like to state that this whole "rubber sheet" analogy is really inaccurate beyond a certain point. In standard GR and most extended gravity theories, we never deal with any "ambient" space, in which spacetime might be embedded in. The braneworld scenarios might be an exception from that. To the best of my knowledge, wormhole metrics arise even outside braneworld scenarios. – Bence Racskó Jan 26 '16 at 18:55

where $A$ and $B$ are two regions of flat spacetime. However the two regions $A$ and $B$ would (in this 2D representation) remain as two flat parallel sheets everywhere with no other joins between them. In effect they would be two different universes joined by the wormhole not different regions of our universe.