Okay, sometimes the soft shell occurs when you have too many views and not enough clicks, thats the way its stated at the dragon cave, and Ive found it to be very true, the rare ones are the silver, gold, paper, splits are not so rare at the moment and the dino's are on the semi rare list! Yoshi is not available to get anymore due to copywrite issues!
Rather than killing your egg if you have to get rid of it, abandon it that way someone else has the opportunity to take it and raise it and get it healthy! White eggs are also rare! I have several eggs right now and many dragons if you guys want to take a look its under my siggy! Any questions you have about the dragons feel free to pm me! When you get a soft shell you should hide it for a while to harden it up again, and also, the first day after getting an egg are critical, the same with the first day they hatch! Those are the days you are most likely to come on and found they have passed away! I killed my first 4 eggs by not getting them the views they needed and have not killed one since, if anyone wants to pm me their hatchlings and eggs I will gladly click on them!
Also, with my silver when it was an egg, someone clicked on it and kept hitting refresh, so I had over 600 views and only 38 clicks, thats when I had to hide it, because some people will intentionally murder your rare eggs and hatchlings! If you have more clicks that you do views your egg will get a soft shell too...