I think the reason they were using TSFs before they decided to bombard in that battle was because they wanted to stem the advance of the Beta and protect the city. They probably did not correctly estimate the number that invaded. If they knew that the defensive line could not be held anyway, they should have just bombarded the enemy from the start. That would whittle the Beta down, but it won't buy time because the Beta don't seem to give a shit if they are being hit. They only pause to kill and eat people.