well...yeah. of course i'm assuming that, because i'm talking about tables where they can.
And I'm not. I've been talking about tables in general. I don't care about what your specific table does. GMs aren't required to use Legendary Resistances every single time the creature is hit by an effect that requires a save, if only for the reasons I mentioned re: weak effects.
In fact, I I've read posts (seriously--google "D&D are legendary resistances unfun") about how the best way to get around LRs is to spam weak powers, and how annoying it is that you have to waste your first few rounds doing that before you can get to the meat of the combat. I've seen people say it's just like the DM saying "no" and that they're too meta-gamey.
i mean, that seems less like rule of cool and more battle strategy. you're not changing the statblock on the fly or making the monster do something it technically can't or anything. but i dunno, maybe i'm just not getting what you mean by "rule of cool" here.
"Would it be cooler for the monster to fail their save or to succeed?"
Sometimes, the most strategic option isn't very cool--let's face it, the
actual most strategic option would be for the BBEG to never let the PCs get anywhere near the PCs,
ever. To only attack them from a distance, or not at all and just send minions, or find other ways to go after them that don't involve the BBEG suffering the risk of great harm.
Obviously, this can be very unfun for the players and Narrator most of the time, since even the most peaceful party or group of players plays games like this for at least an
occasional combat. So we're left with figuring out what the most interesting things to do in combat for the players are, which
is not necessarily what's strategic.
Sometimes, what's fun for the group is to let the monster succumb to the effect, because it can be very un-fun to set up a really cool strategy and have it fail due to poor rolls--or in this case, because the monster uses an LR.
Please note: I'm not saying "don't use LRs." I'm not saying "only use them rarely." Nor am I saying "play the monster weaker than they should be played."
Literally all I am saying is that you're not required to use them every time you possibly can.