I don't know about most games, but I play Defiance, which has a cash shop with XP boosts, and I've never once felt like I was getting cheated if I didn't spend money. I've only spent $5 in that game for extra inventory slots, everything else I got was earned in-game.
As far as getting robbed goes, if I'm playing a game that costs a lot to maintain, I'm not going to mind spending a little bit here and there. I understand that they have to pay to keep the game going, and if I'm playing it, then I deserve to pay something. And you can't forget that MMOs are typically much better supported than other games, which costs money.