Sometimes in middle of teamfight you cast [X Marks The Spot] but can't hear its sound ,because there are a lot of noises in a teamfight.
Sometimes you get lags/spikes and you can't see [X Marks The Spot]'s effect or its sound doesn't play (its a common thing for weak-internet players like me).

Now you don't know to press E again to choose target again
[X Marks The Spot] is already casted and if you press E again, you would ruin your X-Torrent Combo.

What happens:
You have to quickly check the spell in your HUD to see if its on cooldown or not but as you know, if the spell is casted then [Return] replaces [X Marks The Spot], which have not on cd and also have the same Icon as [X Marks The Spot]. (you can't see if the spell is casted or not by looking at Icon of the Spell)
So you only have to see its manacost(80->X Marks The Spot, 50->Return) but it really takes 1-3 second of you to check the manacost.

Having a different Icon for [Return] like Dota1, can really help Kunkka players who have bad internet.