quoting supercell's official response
"Even further, implementing a system that can enable undue pressure on players ("Opt out or we'll kick you" or "Give me Crab Claws or you can't come to war with us") falls far short of this mark."
Not having an opt-in/out is having this exact effect. Participate in war or be kicked, busy for the weekend? To bad, drop all those trophies you've worked so hard to get or be kicked. I understand and agree that if you're in a clan you should be all in but life doesn't always allow this. This very thing almost ripped our well established clan apart. I do agree that giving the leafs the ability to pick and choose may be a bit too much power but honestly if a clan is abusing that they wouldn't last anyway.
my breakdown of the problem
Clan wars are fun however, we are finding them to be a challenge to coordinate effectively for larger clans (ours is often full). Too often members are absent, uninterested, or otherwise unable to participate for one reason or another. This leads to wars being lost, the fun being drained (from war), and precious resources squandered. If that wasn't negative a impact enough, this is also leading to division within clans and two distinct types are beginning to emerge; clans that war and clans that don't. Even for designated "war" clans, the fun is being replaced by rigidity and intolerance.
Suggested means of combating this.
Add a 12-24 hour period before a match up is made for members to opt-in for war. Once match-up begins, you cannot join or leave (the war roster) until war is over.
Give leaders and co-leaders the ability to select who can participate in war via an eligibility flag.
A combination of the two.
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I do understand the fear of that power being abused by the leads. I do also understand your desire for everyone to participate in the wars. It's a great system that you've put countless hours of your life into. That said, you can't please everyone. I think an opt-in would work better, if a player is inactive at the wrong time then they may be inadvertently thrown into war.