I think Ionic Spark is more of a Teemo and TF item honestly. It gives them alot better farm and harrass early game since their range is fairly small and the passive helps last hit minions / harrass champs in the back. Though I think it should be sold endgame since 110 damage every 4th isnt much endgame.
Tiamat is hardly ever used. The reason behind that is 90% because the splash range is sooo small. If they increased the range, it would be used alot more often. The other 10% is expenses. Though if you get good early farm you could pick it up.