If the decoy becomes too successful, the predator will evolve to have more sophisticated detector, find something else to eat, or become extinct.
Modern heat seeking missiles have fancy multi-frequency heat detectors or even IR CCD cameras with image processing to ignore the flares.
Just my opinion. I guess we don't know for sure. There's probably a way for some biologist to figure that out for certain - would make an interesting postdoc.
About the attention-seeking behavior, we can compare it to the same behavior geckos adopt when they find themselves in danger: they start shaking their tails vigorously to attract the attention of the predator to the tail (geckos are known for being able to detach their tails in these cases, and once detached the tail keep shaking vigorously, potentially attracting the predator giving the gecko a chance to escape). In both cases, the spider and the gecko seem to only engage in this behavior once they already have the predator's attention, so trying to escape without shifting the predator's attention to the decoy could actually put the attention on them.
It doesn't necessarily have to be appetizing. Any amount of confusion, hesitation, etc. it can cause is potentially beneficial. Even if only one in a thousand predators hesitates or attacks the wrong spot one in a thousand times when it sees a decoy, that's going to have an effect over millions of years.