I also think a lot of people (myself included) find it hard to comprehend how much money can be made selling stickers in messaging apps. It's mind-boggling, and Snapchat, processing 350+ million messages a day, is in a very strong position to leverage that eventually.
All that said, I'd accept a $3B cash offer in a heartbeat if I were in their shoes. I wouldn't be confident in Snapchat's long-term survival. It might double its user base in the next year and then swiftly die when the next fad comes along. I feel that timing will be critical here. If they wait too long, they could lose it all.