While it may be true for most MMO, Warframe and Path of Exile are two of the exceptions in this case. WarFrame do have micro-transactions but they are mostly cosmetic only and the premium currency (Platinum) is trade-able. You literally never have to spend a single penny unless you want some of their exclusive community created cosmetics where 50% is given to the author and 50% to Digital Extreme. These are the only items which you can't purchase with Platinum, and have to buy directly with fiat currency.