Board Thread:General Discussion/@comment-122.106.190.237-20160329003554/@comment-24668482-20160330070005

If you aren't interested in the rewards and it isn't an artifact collection, then no there is no real benefit to combining the collection multiple times, particularly if the fixers are useful elsewhere. There is a trophy for combining collections, but it doesn't matter which collections are combined for it, and there is a limited number of levels to the achievement so it may already be maxed out in your game. There is also a top 100 category for combining collections if that is something you are interested in, but I'd guess most people don't really care about ranking high in that sort of list, and again if you are using the fixers to combine other collections will still be increasing your overall combined amount.

Really, the main benefit is the rewards, so if they aren't anything you need you may as well leave them until a time when you do want it (or to give the items away to a friend who needs them).