It would've been fine 10 years or more ago but the world has changed, MMOs have changed, our lives have changed, do you really expect to plan my in-time game around the remote chance that besieged MIGHT happen?
That's incredibly unefficient and frustrating.
FWIW I think the nice thing about playing FFXI in 2025 this is kind of a solved problem to some degree. You don't have to personally see the besieged incoming message, you just need anyone in (either of) your linkshell(s), Discord, or unity chats to notice that besieged is incoming and tell you/everyone. Then you can just decide if you want to go do that or would prefer to do the thing you're doing.
If besieged ends up having more valuable rewards in the future, maybe people will say "let's hold off on this seg farm until after besieged is over, I need to get my ISA." For now, I think most people will just say "*** besieged, I'd rather get my muffins/dynamis medals/gil/plasm."
Obviously it's not something you can schedule on your own time and you're subject to the beastmen hordes deciding to attack, but it doesn't seem like something you have to (or can) plan your life around.