As a long time ARES member, an EC, a VE and a repeater trustee I find that emergency communications may motivate new hams, but they rarely are interested in joining an ARES group or ARRL or taking part in nets, drills and events. They are interested in emergency communications to keep in touch with family members or a church group, but not interested in public service.
I frequently hear, " I don't want to join ARES, but if something happens I'm available." That's not the way it works. They will not be trained, will not have disaster service certification and mostly don't know how to use their radios.
The number of ham radio licenses is supposed to reach 750,000 in 2017 (source ARRL). About 160,000 are members of ARRL. That means that only about 21% of hams are members and that is the pool of operators who may join ARES groups.
I had a friend who got very excited about ham radio when his house nearly burned down in a brush fire. His enthusiasm lasted about two days. So, yes, there is an uptick after a major disaster, but it usually doesn't stick.