If you surrender your weapon, the bad guy now has two hostages, and a choice of which to keep, if he so desires.
As for the calvary, even if 911 is called, you are still looking at something like 4-15 minutes average response time for most major municipalities. For example, Dallas was proud that they had dropped below 9 minutes from double digits for top priority calls, even though they are still above 10 for some areas.
http://www.dallasnews.com/s/dws/spe/...as/crime2.html