Full Idea
The main trouble with possible worlds realism is that the only reason anyone has, or ever could have, to believe in other possible worlds (other than this one) is that they are needed, apparently, to provide truth conditions for modals and conditionals.
Clarification
'Realism' here is the belief that possible worlds actually exist. 'Modals' are possibilities and necessities.
Gist of Idea
Possible worlds realism is only needed to give truth conditions for modals and conditionals
Source
Brian Ellis (The Philosophy of Nature: new essentialism [2002], Ch.7)
Book Reference
Ellis,Brian: 'The Philosophy of Nature: new essentialism' [Acumen 2002], p.129
A Reaction
This attacks Lewis. Ellis makes this sound like a trivial technicality, but if our metaphysics is going to make sense it must cover modals and conditionals. What do they actually mean? Lewis has a theory, at least.