I both like & dislike this idea.
I like it because it would add another interesting & efficient way of defending your egg - obby is good but pointless without someone on your team constantly checking nobody is digging it up every so often. This would remove the need to do that, so you can gear up without having to worry so much. It would also feel awesome to blow the egg shell up! It'd almost be like cracking a safe in a bank raid to get to the valuable eggy goodness inside xD (that's probably the most corny thing I've said all week :L)
On the other hand, I dislike it because in short: it completely ruins sneak attempts.
Long version? Think about a situation like this:
Let's say you are playing on Teaparty, your egg is gone and you are the sole survivor of your team. You are facing a full team of campers who are gearing up and have broken the bridge. They have a guard with a punch bow who occasionally checks the chair to make sure nobody is bridging across - so you decide a glass skybridge from the base next door is your best bet.
You manage to sneak inside unnoticed and you go to dig up the egg but, oh noes! There's the undiggable defence (let's call it slime) that needs to be blown apart with TNT. There's no other choice so BOOM, the first layer of defences are gone, but that wasn't exactly subtle buddy. Every single happy camper in the base is now rushing upstairs to investigate the mahoosive explosion above their heads, and when they see yo booty they ain't gonna give you a cosy hug and a warm welcome, oh no no....
Behind the layer of slime it turns out they also put an extra layer of obby, so you can't just smash the egg real quick. You now have to kill every single enemy in the base (while they are respawning and spamming you with their fists) just to stay alive, and then your only option is to block yourself in and then dig the egg. So you boss it, kill them all and block yourself in. But after all that, one of them has the egg cracker kit so you cant dig the egg in peace! They spam hit you over and over until you're deader than a dodo's doornail. Game over :(
In that situation you deserve the win after outsmarting the enemy, but just because your sneak attempt was ruined by placing TNT there's little chance you can win from there.
P.S Sorry for writing so much xD thnx for reading