When this stuff started popping up it seemed shitty but there was one excuse. The games that had it were either free or very cheap, 99 cent kind of cheap. So if they want to charge $1 or $3 for coins or an item and the games fun then sure, it's not so bad. I've bought a few keys on Dota 2 and a hero character in Kingdom Rush. They're both fun games and were worth throwing a few extra bucks at to get more enjoyment from. The thing is a lot of these smartphone games that have it still get criticized. It's not the money, it's the fact that the games are built to force you into paying extra through either massive grinding or insane difficulty.
The biggest issue now is full priced games having microtransactions. These aren't even a few bucks either. Forza 5 and Gran Turismo 6 have $100 and $200 cars you can buy. Oh but, you can ignore it right? Just play the game and unlock them? Well people estimate Forza 5 takes over 560 hours to earn enough to buy all cars without paying. Now lets compare this to Forza 4 which has more tracks and more cars, to complete EVERYTHING including all cars and all races it only takes 340 hours. Still a long time but that's over 200 hours less to unlock more things.
Now how long is it going to be before RPG's have 1 or 2 classes unlocked and another 2 or 3 locked behind a pay wall? Wanna play a mage? Well fuck you, either finish the game on ultra hard to unlock it...or pay us $5! How long until Halo or Killzone or God of War has levels so insanely hard that it forces you to buy extra health or weapons?
How long until you have to pay a dollar to reload in battlefield? Oh, that seems too extreme? Not for EA.
This isn't the kind of thing you can just say "Don't like it? Don't buy it" because they know that and will eventually push and push and push until you MUST buy it or have a terrible time playing. And at that point you won't just not buy the microtransaction, you won't buy any games either.