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.