"You can't have your cake and eat it too."
This is an expression I don't wholly understand. Indulge me, if you will.
If you have your cake, it's yours to eat, and you probably should because if you have it and want to eat it, once you eat it you probably won't want more cake. Especially if you eat the whole thing.
If you have your cake and don't eat it, the cake gets moldy and gross and then pretty soon you don't have your cake anymore and you didn't even get to eat it. Having, as in possessing, cake is not a permanent state of being.
So, I think that people should eat their cake while they have it, and they should enjoy it. Bearing in mind, of course, that every bite of cake better be enjoyed by the cake eater because the cake is being destroyed in the process and if you do want more cake after you finished the first cake, then you ate too fast and you're screwed.
Either way, though, the cake loses. It either withers away to nothing from neglect and decay, or it gets consumed.