This shorthand to create an exciting and engaging game and is no worse than turn based structure.
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The derogatory term "gameist" should be reserved for rules so blatantly unrealistic that their entire reason for existing lies in the rules. Cube shaped fireballs is a great example of this.
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Also realize it is a derogatory term as likely to start edition war fights as not, and should be avoided.
There is a degree of tension in your post, I think. After all, one person's "blatant unrealisticness" is another person's "shorthand to create an exciting and engaging game". If you want the ingame rationale - the flame at the edge of the fireball is cool enough that it is not going to hurt anyone; hence we don't bother keeping track of it for combat resolution purposes.
Yet the reason why the adventurer gets away (unless other creatures join the combat, even if they are on his side) is not because the DM decides that the narrative would be better (although he might certainly believe that, too), but because the rules say that under this circumstances the Tarrasque moves slower.
This was answered by [MENTION=177]Umbran[/MENTION], but I want to state my agreement with him: a rule that says that, as the number of PCs in play drops, so the fury of the NPC/monster drops, is a rule for pacing the action, and managing climaxes and denouement within the context of a combat encounter. You may not like such mechanics, but they're hardly revolutionary in RPG design. Legendary actions are one version of such mechanics, designed to fit within D&D's combat turn structure.
If you want to rationalize it that way to yourself fine. Too bad that it doesn't work that way with the Sphinx.
What are you talking about? As the sphinx defeats its foes, it first stops casting spells against them (3 actions required), then stops teleporting (2 actions required). So we reach a climax, of the sphinx facing off against its final foe, claw vs [whatever].
That's a pacing mechanic that is apt to push the story in a definite direction. You might think it's a poor story - in which case you'll want to rewrite or not use the sphinx - but that doesn't change the fact that it advances a particular story.
Why can the Sphinx only teleport when enemies are around but not when it is alone?
That might be true at your table. At my table the sphinx can teleport to its heart's content when it is alone. The reason for putting the teleportation ability into the legendary actions is to give the GM advice on what is the best way to implement Sphinxish teleportation in the context of a fight between the sphinx and PCs adjudicated within the turn-by-turn, round-by-round combat resolution system.
take the Sphinx which is prevented from flying and caught in a wall of fire or similar effect. It spends its move trying to get out but does not manage it and thus takes damage. Now the attacking wizard casts another spell and suddenly the Sphinx teleports out of the fire. Why only after the fire burned it and not before?
Because it took time to muster its magical energies? Because teleporting is
hard, so it tried to move out naturally at first, but then when it found itself unable to, and starting to burn, it teleported instead?
Use your imagination!
I suddenly want to run a campaign where some horrific thing arrives from the lower planes or from the stars, and the only chance humanity has is for someone (i.e. the PCs) to find and wake the tarrasque to fight it.
Now this the best post of the thread, and may well feed into my 4e campaign, which is at about the right level for the tarrasque! (But sorry, the XP well is dry at present.)
