What is the benefit of using Futures over parallel collections in scala?

Futures become useful as soon as you want to compose your deferred / concurrent computations. Futures (the good kind, anyway, such as Akka's) are monadic and hence allow you to build arbitrarily complex computational structures with all the concurrency and synchronization handled properly by the Futures library.

I think the main advantage would be that you can access the results of each future as soon as it is computed, while you would have to wait for the whole computation to be done with a parallel collection. A disadvantage might be that you end up creating lots of futures. If you later end up calling Future.sequence, there really is no advantage.

Parallel collections will kill of some threads as we get closer to processing all items. So last few items might be processed by single thread. Please see my question for more details on this behavior Using ThreadPoolTaskSupport as tasksupport for parallel collections in scala

Future does no such thing, and all your threads are in use until all objects are processed. Hence unless your tasks are so small that you dont care about loss of parallelism for last few tasks and you are using huge number of threads, which have to killed of as soon as possible, Futures are better.