I think a lot of people are looking at this the wrong way. Everyone sees this as: "Well, no need to build towns anymore. Its all going to get destroyed permanently anyways". Kainzo has already said that he is working to see if the TNT Regen would work in this case. I think no one really wants war to permanently destroy blocks, PVEers and PVPers alike.
Even if TNT wouldn't regen, I believe that it'd still be ideal to cause the blocks that are destroyed to regen. Like what everyone needs to understand is, that most likely the end product of all this will only leave it so that your un-LWCed chests will be at danger, nothing else.
The pure intention of this is to give players a way to actually raid other towns, instead of sitting outside a town praying for someone to run out. It also gives the players that don't want to join war the option to just pay for neutrality, which I'm sure isn't really a big fee for large nations/towns.
In response to chesterH's cons (more specifically the lack of private retreats), private retreats used to constantly get griefed either way because there were 0 protections anyways. The only way to truly solve this problem is to bring back personal regions, which I believe might be coming back soon (depending on Kainzo's decision).
TLDR: The ideal situation that even Kainzo agrees on is for TNT to be used in raids, and to allow the blocks destroyed by the TNT to regenerate. With this, NOTHING will be permanently destroyed, except for unprotected chests (but even that might regenerate). So rest assured everyone, Kainzo is trying his best to let blocks regenerate.
(Reminder: TNT would be hard to attain for the fact that dungeons exist, and that the amount of creepers that exist in the world are quite minuscule. So you won't get pelted 24/7 with TNT.)