This kind of beasts can add a respectable degree of flavor, wonder and playable depth to a campaign setting, and even then they are not that usual. Probably one of the causes is our own perception: the biggest thing we can find today on land is an African elephant and any thing of that size or bigger comes to our minds as something to awkwardly and slowly level everything on its way, like a dinosaur, a giant or the tarasque itself that, as Nirkhuz points out, fucking-kills everything in its wake.
But a good part of that is linked to our perception as land-based beings: something that big that also has to be supported by earth defying gravity is doomed to be awkward and slow. But that doesn't have to be true in other enviroments such as the sea or the sky. After all, a dragon is always much more graceful in the air than in land.
That was one of the things I think I took into account when writing Criaturas del Vacío Celeste [my bestiary of beings from floating islands]: I tried to fill it with creatures that are simply too massive to appear in any other place. Some of them are even legitimate buildings like the machine island or the gelatinous pyramid, or islands like the spheric turtle or the geo-dragon.
And I think I fancy them because, as I say, they add more texture to a campaign setting. The fact that one can live on top of a living creature raises many questions about it's relationship with its inhabitants or, on a more tactical level, a 165 feet tall pyramid made of corrosive jelly and with tentacles is going to require a lot more planning and unconventional resources to be defeated. Just direct violence won't cut it.
And really I think they shouldn't be a challenge just for advanced players. In fact, the other day I was discussing that, if one of these days I started a new fantasy campaign, the first encounter would probably be a giant, to make it clear from the beginning that there are usually challenges that cannot be resolved just by violence, that sometimes it is even reasonable to run away and that I am a merciless jerk that likes dead characters. But that last point goes beyond the scope of what we are discussing here.
It would be cool to run a campaign full of giant creatures roaming around or even one where the players fight mostly or only stupidly big things (hey, it worked for Shadow of the Colossus) because it makes you think a lot more about the logistics of such encounters: how to climb them, how to make them fall, where their weak points are... And about the nature of the world: how do you build a city in a world where the average monster can demolish a wall as if it was nothing? What kind of weapons and armor have they specialized into? Who makes all that giant-thing-slaying gear? What role does it have in their cultures, their daily lives or their way to make war? And, again, those are some of the questions I asked when I imagined a campaign played using Criaturas del Vacío Celeste.
But what I want to say at the end of it all is... take the tarasque out and as soon as possibl. Thanks for reading. Valmar Cerenor!