If you don't have people on the outside then the people on the inside have to work twice as hard to put out the fire. You need people doing both for a complete fire attack. You need people on the outside watching for structure collapse and other issues for the people inside. These people are also on the ends of fire hoses.
I don't think either one is more important then the other. It comes down to scene size up. If you take an offensive attack or a defensive attack. If it is small you go in. If it is big you start out and then go in.
I can't think of a more absurd question. If you don't have folks on the exterior you get no ventilation, you get no water supply, you get no command, you get no RIT. If you don't have folks on the inside all you will ever do is make basements into ponds, you save no lives, you satisfy no customers. Everyone is important on a scene all the jobs are important and need to be performed.