The chain of permissions goes from Object permissions which override Category which override Global permissions. If you have no global permissions, you have no fall-back. To me that is a broken system. There must always be the final answer, even if that answer is no.
In other words if you forget to set object or category permission then what is your Tiki supposed to do? You should always be able to predict exactly what your system is going to do, if you have to guess how it's going to behave then your system is incomplete.
I hope this doesn't sound too preachy. I certainly don't claim to have all the answers, however to my mind you want to think of permissions as from least specific (global, works for most users) to most specific (object, which only for a very specific set of users).