I have actually seen Paizils argue that, as an extradimensional space, you can't teleport out of it. Which may in fact be true, but just makes the spell completely ridiculous. Becomes a "have flight, or GTFO" ability at that point.
EDIT: I suppose Helpless Prisoner doesn't technically make you worse day-to-day and is just literally useless instead of 3E toughness which is "useless." When I say a feat is worse than useless, I work with the assumption that someone who takes it actually wants to / tries to use it. I really don't think anyone intentionally declares, "My character doesn't NEED feats, so I'm going to take this one that does nothing!" Now, there is the "spell mastery argument" (a DM of someone w/ SM gets the idea to steal the spellbook / thinks its less harsh to do so and is thus encouraged to do so; so for a PC w/ helpless prisoner, the DM is more encouraged subconsciously to put the PC in a prisoner status), but leaving that aside... If a PC *were* captured, and *did* have this feat, and *did not* realize how bad it is, he would probably try to use it. And then the feat would become worse than not having it at all.
That was my reasoning.