Having you be able to accept/decline would kind of defeat the purpose. It's not a war if you both agree to it. You war people you have disagreements and problems with.
I personally love guild wars, it's a chance for my guild to test itself against others. And i enjoy it quite alot. If you don't want to take part in a guild war, simply leave the guild and join later when it is over, or go hide in the guild castle/your house/seldomly visted areas/etc.
I'm the guild leader lol, and it's a stupid suggestion to just leave the guild until it's over, I just don't want to deal with it at certain hours especially when it's pushing Midnight or past Midnight. And it's more of certain guilds are more likely to take advantage of it at hours when they have more members on etc, if anything it's an unwanted distraction when you're farming and you're already tired.
Besides, I see guild warring as more of a way to bully other guilds if there isn't a Accept/Decline. It's not like money is ever an issue to a number of guilds, besides what's the point of being on a PVE server if you're forced into PvP.
Originally I was planning to do some HOS/ZS/DL runs for the nubs after siege but we got battled during the middle of the HoS run and got camped on our way out. We had a bunch of underleved people with us and it was just annoying if anything, it's fun if you have nothing to do and you were looking for something to do.
But, when you're in the middle of helping your guild etc it's stupid, I'd rather be helping my guildies out by getting them some basic gear than dealing with a 1 hour battle with X guild following us.
The other thing is that there wasn't really any problems, they hadn't had siege and just wanted a siege replacement. We had siege, why do we have to deal with other guild's problems?