The curator who shows up may very well be doing 'a lot of work'. However, the author has no way to know if this is the case or not unless he or she actively follows the movements of said curator, and chances are most authors don't follow every single one. Most of the time, an author is more likely to do so if the curator actually puts time into the curation effort on his or her post specifically, or if the curator happens to be a dolphin or whale.

And if there were no curators (suggesting, it seems, given the size of Steemit at this point, that curation itself becomes non-existent, failing which there would always be people available to comment on, upvote and resteem content), then one of Steemit's central, natural functions would be rendered deficit, and compromise the inherent integrity of the system, and possibly collapsing it as well.

And as for the work curation entails, Steemit is a free, open world to rove around in. How much work a curator puts in is up to the individual curator. Just because one is a curator doesn't automatically mean they put in great amounts of work into the process. I'm not saying there aren't curators who do spend noticeably vast sums of time picking through the agglomerate repository of content here in pursuit of the 'gems' you spoke of, and who contribute extremely meaningfully to the community. Hell, my Steemit Debut post was picked up by one such individual, to whom I am hugely grateful. That said, there are curators who simply upvote and drop one-word comments namesake for curation. While I would love to be allowed personal control of the payout for each curator who looks through my work and provides input (maybe that could be the trick...a slider that allows the author to choose how much each individual curator on his or her post gets within the range of the overall payout), I simply believe that those curators that actually contribute, deserve everything they get from that 25%, and those that don't, well...the limit's there for a reason. Even if the curator does spend a lot of time fishing for the gems, it might not always be the same author's gems, so the former has plenty of other places to look for gems besides that one author. And if the curator does contribute somewhat singularly to one author's post, chances are still high that his contribution won't necessarily match the effort put into the post itself. In either scenario, however...the author in this example has put in a great deal more work. This wouldn't apply as much relatively to an author who just uploads a picture and quick caption for a majority of his or her posts, obviously.

Also, I might possibly be seeing only one side of the picture here, because while I do curate all across the board, delving deep into whatever pockets of posts I can find here on Steemit, I'm still only a one month old minnow. If there's something I'm not seeing, show it to me, because all my posts have usually had some thought and planning go into them, and the curation only seldom complements the post or its content as well as I'd think it should.