After reading Greene's book I am less inclined to believe in the reality of parallel universes. The idea of everything being repeated an infinite number of times in an infinite universe seems ridiculous. Infinity is a mathematical abstraction that has no correlation with the physical universe. All discussion of infinity immediately bogs down into paradox, which is a sign that you have gone down the wrong track. String theory seems to be an exercise in devising a mathematical model to explain an imaginary universe and then trying to prove the imaginary universe is the real universe because it makes sense to you. Nine spacial dimensions? Strings or branes instead of particles? Why not?The many worlds interpretation of quantum mechanics just seems to be wrong. Quantum theory explains events in terms of probability, but if every probability becomes a reality in a parallel world then the probability is eliminated. If one outcome has a likelihood of 90% and the other 10%, but they both happen in parallel worlds, then all possible outcomes were 100% certain and probability is meaningless.
The holographic multiverse theory is just a speculation built on top of a speculation, while the simulated and ultimate theories appear to be little more than philosophical games. The inflationary bubble universe theory seems to be the only one of the multiverse theories that actually has a decent argument behind it.