The simplest defeasible claim in 2013, is still: "we don't know".
Consider, 'How does the sun come up every day?'
Answer A: God makes it happen.
Answer B: The earth is a spinning geoid that rotates eastwards, once about it's axis, roughly every 24 hours. However most of the effect is not related to the orbits of either the sun or earth, with respect to each other. The sun "rises" is more an observational effect of a stationary observer on the surface of the earth, having their field of view rotated.
(For the record, answer B can be improved of course, though that's not the point here)
"Simplicity" does not mean a catchall (Answer A). It also does not necessarily mean linguistic brevity either. There is debate around what it means and it's beyond the scope of my comment[1]. This is the problem with string theory, that also tends to be popular among the MWI crowd too. It's overly broad and the math looks pretty, but those are not sufficient conditions for a good scientific theory. (Oblig xkcd [1])
MWI, like string theory, makes little if any falsifiable and testable claims.