Perhaps the sunk cost fallacy is useful because without it you're prone to switch projects as soon as a higher-value project comes along, leaving an ever-growing heap of abandoned projects behind you.

Sunk costs might in fact be useful heuristic for many people. But if rationality gets you in trouble in this area, use more rationality! - after all, once you know about this switching behavior, you can consciously counteract it with strategies (precommitment, etc).

What you're pointing to is that a little rationality can be a dangerous thing.