clearing messages when you leave the chat window will make people not use it
With WhatsApp picking up speed in my close friend circle, Facebook messenger being used for everyone else, and the occasional Google hangouts messages when I feel like being online, I don't think even taking the "ephemeral" approach is going to make a difference in persuading me to start picking Snapchat over competing platforms. Snapchat is fun because as other users mentioned, I get to send bullshit one-off joke snaps to my friends. I don't want to maintain a conversation of any sort through another app. However, knowing the conversation is immediately cleared further dissuades me. The point of this feature is that it's good for non-committal "conversations" and very easy to engage/disengage. But I already have WhatsApp for that, and I have group conversations there, and my messages aren't deleted.
I use Snapchat for both very close friends and acquaintances too, I can send both inside jokes or just a simple hello to someone I haven't spoken to in a while. I don't feel an obligation to respond to snapchats and don't feel ignored when someone doesn't respond to mine, how I would via text, or facebook messenger.
I personally never use facebook messenger. I've had it turned off for a couple of years now and only use it on the rare occasion that a friend changed phone numbers (friends moving abroad mostly) and even that is just to get their new number so I can use whatsapp or text messaging. As for whatsapp I strictly use it for large, on going, group messages or friends in other countries. But I guess everyone has their own use-cases.