If you look at WestPhilly's and Capcitiy's comments earlier, they talk about not going in with a handline for S&R in urban departments. Back in the day, the truck/squad didn't bring in a hose, but that misses a critical point - they were usually second in after an engine. The truckies may not bring in a line, but they are working with engine companies who are (hopefully) fighting the fire.
Rsk vs. Reward - plain and simple. We try to bring in the primary line in with us but if it isn't there when you are ready to enter the structure then you do what you have to without endangering yourself or your search team members. Fire conditions, type of structure, time of day, confirmed people within, etc. , all this info will dictate on how much you are willing to go without endangering yourself or the search team. Ultimately yes we try to search off the line but thats not always a gaurantee.