Well, I use Gnome Shell with 4 desktops in a very similar way.

But the KDE activities, if I understand it correctly, are named and you are kinda expected to create a new activity for any significant task (holiday planing, upcoming talk preparations, coding project foo, etc.) and then remove it once you no longer need it.

But in practice one is usually too lazy to organize stuff like this all the time, so the holiday planing just ends on the "non work desktop", the coding stuff on the "coding desktop", etc.

So basically generic desktops vs desktops for specific tasks.

But as one KDE developer noted on a Plasma Mobile talk I'have visited, activities make a lot more sense for mobile devices - as with the holiday example, you would usually have the mobile device on hand during the holiday, so having various kinds of information ready in a holiday activity makes sense & should be worth the hassle of organizing stuff.