Star Wars: The Old Republic, but they're also sometimes very limiting. Depending on your class and spec, you can find yourself with only one or two companions with useful abilities. But that's partly by design; according to the latest community Q&A, the distinctiveness of companions is both a result of their abilities and their personalities. The team has no plans for making companions more interchangeable, at least for the moment.
If you were hoping for better news, don't fret, as the other answers are a fair bit more positive. The technical hurdles preventing some classes from having a modifiable offhand are being looked into, and the answers also delve into the details of adding a new augment slot to existing modifiable items. There's also a discussion of the different rates of gear acquisition for players on the PvE and PvP progress tracks, along with a variety of other answers for players of several stripes.