Rob and Carrie don't get a whole lot of screen time, but that doesn't mean they aren't important. While they serve as minor catalysts that help forward the plot, they more importantly serve as a foil for Clem and Joel's relationship.
Rob and Carrie are pretty endearing. They care about Joel—though maybe they care about him in different ways: Rob wants to give him the straight truth, while Carrie wants to save him from the pain. Either way, we know they are Joel's friends and have his best interests in mind, and so we like them.
But their relationship itself is definitely not hunky-dory. The first time we see their troubles comes when they have to make the big decision whether or not to give Joel the Lacuna letter about Clem. During their discussion, Carrie gets very upset; she throws laundry on Rob and storms upstairs. She also tells Rob to "give it a rest" when he suggests they smoke a joint while Joel is upset. Sure, this may not have been the most sensitive solution, but Joel was only trying to help.
While none of their other arguments is all that serious, there's rarely a scene in which Rob and Carrie are a perfect, loving couple. The next time we see them, Joel is still upset, and Rob is loudly banging away on a birdhouse. Carrie again wants him to "give it a rest" which makes Rob mad because, hey, the dude just wants to build his birdhouse. And so on.
These are petty things but the point is that despite the fact that Rob and Carrie don't always get along, they still know each other. No one's been to Lacuna recently (at least as far as we know), and they seem to be able to deal with their small arguments and still be a successful couple. They seem to be in the film to remind us that there was a time before Lacuna, which means that Lacuna is not an answer to all our problems. As Joel begins to think, maybe it's best to work things out instead of taking the easy route and forgetting everything.