... have you been drinking?
Yeah, in the real world if you were a local at a bar or on a ship or in any social situation of any kind you would meet all sorts of people who have all sorts of level of engagement and interest in the society around them. A world like Arcanum feels real (despite its awkward and sometimes frankly bad writing) partly because you have companions have varying levels of engagement with you, each other, and the world around them. Companions with "their own goals and different motivations" doesn't amount to much when every arc proceeds in adhesion with the exact same formula. The reason why dramatic fiction has to be written with supporting cast of varying importance and participation is because the audience is supposed to experience things vicariously through the protagonist and the supporting cast have varying importance and participation to him. When you make every character equally important and make their arcs proceed according to the same formula, it feels far less like you are a discovering an amazing fantasy world and more like playing a video game that somebody designed in a very obvious way.
Furthermore, in Planescape, when you think of companions, you don't think of them it in terms of concepts like "personal quests." Grace's personal quest (if she even has one) happens before you recruit her (talking to the prostitutes in the Brothel of Slaking Intellectual Lusts), you would never know Dakkon even had a quest unless you were a mage and spent a lot of time digging through text, Morte doesn't have a quest at all, etc.
Gold Pact Knight as described is a great role playing concept for a variety of reasons. The opportunities for interactivity are endless because unlike say Durance or Eder this is a guy who has a plausible reason to know everyone and be involved in everything (he's a mercenary who has been around the block for years or decades). He knows secrets and can serve as a compass to any number of treasures or Easter Eggs (the vault he was once employed to guard he can put out to the player). His motivation is probably the most natural and sensible of any companion given the nature of the Watcher's quest (which involves killing stuff and finding valuable treasures), and can be used to turn traditional concepts of heroic fantasy on their head (raising an eyebrow at the suggestion that he should have a personal quest for the Watcher to solve, "I'm on your dime, boss." It's pretty clear why people would like him and want to have him in their party on at least one play through. Furthermore, his participation is based partly in mechanics and economy rather than purely story line reasons.
