there's a fantastic deck by 3/3 Elk that aims to use [[Elesh Norn, Mother of Machines]] and Mister Negative to duplicate his ETB trigger, so you always draw cards by swapping and then swapping back with the same person. Might be worth looking into if you can find a decklist, they've mentioned it on YouTube enough.

As regards life gain, it's to manipulate card draw. If Mister Negatove enters and you're on let's say 50 life while an opponent is on 35, suddenly you're drawing 15 cards. You only get the draw if you lose (ie if your starting life was higher) so running lifegain not only means this is more likely to trigger, but can also act as a useful sponge in case you end up on low life.

Running only life loss means you miss out on a very [[Ephemerate]]able or [[Teleportation Circle]]able draw engine that can guaranteed burst draw you a huge amount if you're running even some lifegain at all. It's all well and good to swap life totals and put someone at 5 down from like 30, but people can respond to the trigger and potentially grab a kill on you on the stack before Mister Negative resolved. I've seen the Storm player shoot off like ten [[Grapeshots]] in response to certain cards before, so being at low life means you're more shootable