I've tried buying XP a few times and I never feel any significant difference. I see your point, Epi, but:
- "That means that a 20% boost of experience is a little less than one level's worth, from the start of the game, by the time you get to level 10."
From the start of the game! Which is never the case; XP1 is at best bought at level 2 or 3, and XP2, around level 5. So you will never realisticly be a little less than one's level worth at level 10. And all along the game, the little more xp you're accumulating is really not significant.
- Between level 3 and 8 (let's say), you will be, skill being equal, unable to deal with a DG with, for ex, Nirmoth & Unbreakable; you'll be less efficient to stand a lane (less WS), to farm (less xp and gold) and maybe unable to defend a tower against one or two DG.
- Even if you finally got one level more than opponents (between level 11 and 13). It's not such a big deal imo. It's not a fact that is going to reverse a huge battle.
- 3600 / 250 = 14,4. What about, on cataract, buying 15 locks to always keep central flag on your side? Advantages: you can easily divise the costs between allies, and you can buy locks from time to time and not in one shot.