The thing is that, at the time of Pink's "shattering" they had NO IDEA that you could reform the fragments with other Diamond abilities. The Diamonds never even thought about using their powers like that. SO the assumption was that shattering = death. Also, we don't know if there's a limit to how easily you can reform a gem. If the gem gets ground down to dust, is it still possible? We also already know what happens if you don't have all the pieces and it can be horrifying for the gem (the fragments you see with the Cluster).

Secondly, there was a war, yes. But the Crystal Gems were not shattering anyone on purpose. Rose even had Bismuth make a special sword that would poof but not shatter gems. I don't know what the Diamonds were doing but we do know would put suspected Crystal Gems into devices and try to extract info from them (Lapis). There could have been shattering or torture in other ways that we're not aware of.

We also have to remember that EVERYONE but Pearl thought the shattering was legit. And having a person who is essentially a god in your society suddenly get killed is totally unheard of. The Diamonds are suppose to be perfect beings with no flaws and no weaknesses and everyone in that society is immortal as long as their gems are fine. I'm more than willing to bet that a Diamond has never ever been shattered in the history of their people and for good reason. Diamonds are very, very powerful on their own.

Also, we have to remember that this is a story. We, as the audience, are usually going to have way more information about what's going on than any of the characters at any given time.