As of this point, I have no clue why if you run out of range, your fairy is dismissed. What should happen is when your fairy gets to max range, it automatically switches to Heel. There is no reason why this shouldn't happen, because there's no situation where you would rather resummon your fairy than to have it follow you. And if there is, you can simply dismiss your fairy by right clicking on it.

The only argument I can see is that some may say it makes you pay more attention, and while that's true there's also nothing wrong with a convenience feature that isn't a buff of any kind.

What say you?