Our SU hosted world thinking day this year and only 8 troops (in including mine) had a booth, but 20 troops signed up to attend. That meant that the 8 troops "hosting" had to supply enough activities and materials for close to 200 girls. My girls had fun but were exhausted and a little disappointed because they put a lot of effort into their booth and didn't see the same effort from the majority of the attendees.
It's hard to keep 9-10 year old girls motivated when they see themselves always giving and get so little in return (except complaints from other leaders when we had to limit it to 1 craft/swap/goodie per Girl Scout, none for siblings or other tag a longs).
I don't know that the SU events are required. I know they are STRONGLY encouraged...like cookie sales. A girl does not have to participate in cookie sales, but the council will punish a troop that does not participate by not allowing any other fund raising activity if they fail to sell cookies.
The bottom line is that this is a mostly volunteer organization. If you make it difficult on volunteers, you will lose them. They are already hurting for troop leaders...I am under constant pressure to make my troop bigger because no one else in my town wants to be a troop leader for my age group. (We have 3 or 4 brownie troops - Grades 2 and 3, but I'm the only Junior troop Grade 4.). There are nightmare situations where a leader who can't say no has 40 girls in her troop...and all they do is color worksheets. The parents complain, the leader has begged for help, but no one listens.
It's just really frustrating for everyone in the organization I guess. I've promised my girls we will have a troop for as long as my daughter and at lease 4 other girls are interested. But most girls drop out between 5th and 6th grade. So I guess we'll see.