I'd like to speak briefly about something else "between the lines" of that article. I'm sensing underneath the terms "underused" and "underdeveloped" a feeling of dissatisfaction for how Anya and Tara were incorporated into the circle of core scooby friends, and dissatisfaction for the extent of attention and development they received. So some words on that:
I don't have a problem with certain characters being peripheral. That's real life, among some friends we all have felt as if we were periphery, and that's fine. I have been among friends who knew each other longer and shared more experiences over many years, and I knew I couldn't really "break in" in certain ways, which made me realize I was somewhat peripheral among them, but I was fine with that. And vice versa, I have a few extremely close friends I've had since being a very young boy and we share endless bonding experiences, and among those friends, a newer friend will probably feel a little more distant. Friendship circles are concentric, circles within circles. Not all friends are as close as others. But that's not necessarily a big deal, and that doesn't mean we don't appreciate or love less close friends in profound ways sometimes. I like that the show reflects life in that way: Anya and Tara are the new friends, so it's natural they are not as tight to the original core group as those core friends themselves, and never could be. It's also natural that they found their way into the group through one particular member, and it's natural that they are much closer to that person (the girlfriend/boyfriend) than the others. So for example, Buffy showed much less sorrow at Tara's death than Willow, but that completely makes sense to me. Truthfully, Buffy really wasn't that close to Tara. They shared a few poignant moments, but I never got the impression they were close friends at all.
Why can't Tara and Anya be what they are, in the context of this position? They aren't the focus of the show, they don't get as much attention as the core members, but why does that have to be a big deal? That makes sense, concentric circles remember? They were never the center, they were never primary. When Clem doesn't get any attention, no ones says anything. He is a minor character of course, so he gets about what he deserves really. Tara and Anya are of course more important to the show than Clem (and therefore deserve and require much more time and attention) but they were never Xander or Willow or Buffy or Giles either. They deserve way more attention than Clem of course, but not as much as the core scoobies. Circles within circles. So on the issue of how much time or attention was devoted to Anya and Tara, I feel the balance was about right. They had their poignant moments and screen-time and even full episodes of being the center of attention, but at the same time they were never the focus. Just like life: in this particular group, they were somewhat peripheral.