LoL is capped at lvl 30. With boost, it will take you 1 week instead of 2 to reach lvl30. It does have any in game effect. Furthermore, they are not an item that you are trained to use as a currency.
The South Park episode about freemium game is very good at explaining how the freemium market model is based on addiction.
XP boost have no level after lvl 30, which is reached quickly, and is where the real game begins.
IP boost allows you to buy runes quickly, this happens outside the game. Runes have in-game effects, but the difference between T2 (which are extremely easy to buy without spending a dime) and T3 are minor.
I played LoL for years without spending a single penny in it, and I never were under the impression that the game was purposely made boring to force me to buy in-game items.
Freemiums game feels that way. At first, everyting is fun, and they even give you some 'free' currency to hook you on spending, and after a few hour, if you don't pay, the game becomes boring as fuck, because everything take hours.
The South Park episode about freemium game is very good at explaining how the freemium market model is based on addiction.