It won't be necessary, but many people will use them. I will likely use one since I find Samsung's look to be a little cartoonish for my tastes. Their theme manager is nice, but I don't care for the all or nothing choice. If I choose one theme, I get their color scheme, icon pack, and wallpaper. I'm guessing I can change wallpaper again, but the other things are locked. I'll use a third party launcher so that I can change it all to be the way I want.
As far as a concern with lag, new devices today can easily handle a third party launcher. If you experience lag with one, it is likely an issue with that specific launcher itself, but the main ones are all pretty good. Personally, I've used Nova and Action Launcher with a number of devices over the past years and have not seen a noticeable affect on battery life or lag.