The thing is, that a well supplied base can survive for prolonged time, no need for a shield. A base with 10k of one kind of repairs and ~20k of h-fuel can survive for damn long time. A whole day of assault even.
EDIT: btw, defending a base is still a lot easier than attacking one.
With your suggestion, bases will be impossible to destroy once they have shields, 5k of H-fuel can keep the shield running for ~5 hours, do six runs with 1 transport, daily each time with 4k of H-fuel and 1k of BA and your base is impossible to be destroyed. That quite ruin the group effort, and the fact bases should be rare
EDIT 2: Not to mention the fact you can use MOX and then the base will consume only 2/3 of the fuel. 5k of MOX is about 7 hours of shield.
Good. They should be darn near impossible to destroy given how much effort is required to build them. Why should bases be rare? So you can play with the new toys and everyone else can't?