Q. Why would it be an awful idea for gateways to pass broadcast packets between networks? What would be the merite of doing so?
Answer: All broadcasts would be spreads to all networks causing a lot of network traffic. If broadcast traffic were restricted to important data (and very little of it) then broadcast propagation would save gateways from having to run special software to watch for this data (such as network routing information) as well as rebroadcast it.