I would not have much a problem with split-screen, except when playing PvP, because I've had a lot of experiences in team based games, where people with guests are bad, AFK, or don't even try, beacuse of messing around with the guests or vise versa, which can ruin the experience for people trying to win in pvp. But other than that It would be great to have split-screen for PvE and other modes.
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The way Destiny is set up, there would be no way for a Guest account to play with someone. Both players would be required to have XBL Gold/PSN Plus to play splitscreen. That pretty much solves your problem lol.