The accuracy requirements for e911 are quite low and are given in terms of a target average. From Wikipedia:

> Wireless network operators must provide the latitude and longitude of callers within 300 meters, within six minutes of a request by a PSAP. Accuracy rates must meet FCC standards on average within any given participating PSAP service area by September 11, 2012

As such the location provided by a barely-compliant mobile phone provider would only have a 50% chance (depending on distribution and what average means exactly in this context) of being within 300 meters of the actual location of the call. I don't know what the real world numbers look like, but it seems very reasonable that the emergency service personnel would need much more accurate location information than that in order to respond without a lot of searching.