Question

Go watch Fanatic's game on day 2 of EU LCS and you will see why.

Imagine having Vladimir is allowed to take ignite for early lane phase, ok then he gets to farm because he actually has duel pressure, now your farmed Vlad goes home and switches to TP after his first or second shop so he doesn't lose map pressure.

Or, your ignite Vlad gets ganked early on so he has to burn flash. Well damn, let me go back and buy. Oh yeah while I'm here I'll just trade this ignite in for Ghost so I STILL have an escape when I come back to lane.

Or you can do what Rekkles did and have your ADC take tp and then switch over to Heal only when they were going to team fight. Here's what they did and imagine how damn pissed you'd be as a top laner if this happened to you. You and enemy top laner both burn TP to get back to lane, so map pressure appears even. Then you guys go back to 1v1 lane, enemy top laner backs to buy his first built item and you do the same. While both of your TP's are on cooldown, the freaking enemy Sivir from bottom lane teleports right into your frozen wave and pushes up to tower and take's 90% of it's hp while you are having to walk back to lane.