Well, if you already own a old game, you don't need to give any money to GOG.com & Co. just to play it, as you can do an easy search and figure out how to set up DosBox.
However, if you do not own a game you want to play, you need to buy it from somewhere, right? In this case, it is preferred to get it bundles with a tested DosBox setup - not quite everybody who likes old/classic games knows how to setup DosBox, or even what is it all about.
While it may seem that GOG.com & Co. are selling old repackaged stuff for shiny new $$, that old stuff is not quite free for everyone to take. It was made by someone, and someone still has property rights on it. While that someone may not track copyright violations & stuff, that does not change the fact that the old games sold by GOG.com & Co. are still someone's property.
(
BTW, GOG.com has several free games in it's catalogue, but they are free because their creators either made them free from the start, or the owners re-released them free later on)
But I feel that is nothing wrong to pay for someones brilliant work, no matter how old is it. Yes, GOG.com & Co. makes a profit out of it, but considering that some of those games are hard to legally buy elsewhere and also considering how little you pay for a game to own FOREVER (no in-game payments for these titles), I believe it is worth it and I find nothing wrong in GOG making a small profit out of it.
What bugs me is that GOG.com does not own the majority of the games they're selling (except for the Witcher series IIRC), and the copyright holders for a lot of the others are not the original creators of the game, meaning that you may end up paying a company that has nothing to do with the creation of the original gem you're buying.