When is phantom dancer a good pick over statikk shiv?

My gut tells me never.

The fact is that the +10% critical strike chance and +10% attack speed doesn't provide as much damage as shiv procs do. I have done the math myself, and shiv is better in any of the following circumstances for damage output:

• You are fighting someone with a ton of armor, so that the worth of the magic damage increases far beyond the worth of a slight % increase in your physical damage per second, despite accounting for last whisper.
• You are chasing someone and only manage to land one hit every so often, so that the shiv procs on every other auto attack.
• You are in a team fight where the shiv can chain to 3 other people.

Any one of these might be a niche circumstance, but taken together it pretty much exhaustively covers every circumstance you would be fighting in.

You pay 300 gold and lose 1% movement speed to gain the ability to move through units. So you are able to chase through enemy minion waves, and enemy champions to stick to a target, essentially. The problem is that ghost also gives you the ability to move through units.

You might be thinking "You could take shiv and ghost, or you could take dancer and something else, so thats a choice".

Its not really though. The movement speed you lose by not taking ghost and shiv makes diving without it pretty much impossible, so the fact you can move through units passively is moot.

Phantom dancer should just grant more movement speed than statikk shiv. Or maybe Phantom dancer should have an active that grants movement speed, or maybe even a stack system that works the same way shiv does that grants you a huge very short lived burst of movement speed on your next auto attack. Or maybe something else. Just make it worth buying sometimes; the alternative is currently superior in any practical circumstance, and not having a choice isn't a good thing.