There have been pros and cons to the relationship. Briala's insights were beneficial to Celene outmaneuvering Gaspard in the confrontation with Tegan, for example. Celene has also used the allure of a possible marriage with her to her advantage, which was brought up early on.
Xil likely has her own reasons to add in the myriad of reasons that she may prefer Celene and Briala. They are two strong, intelligent woman in their own right, and an interracial relationship by human/elven terms. They've helped and hurt each other; there's a lot of interesting story to explore in that respect. I think a successful lesbian relationship may be part of the appeal of Celene and Briala as well, in contrast to what we've seen with the more sadistic lesbian relationships with Leliana and Branka.
I'm not sure you can call it a successful relationship when it's over, and you yourself suggest it would be extremely unlikely that they will ever get back together, romantically.
Edit: And while I appreciate your thoughts on the matter, I really was looking for Xil's opinion here. She has complained about the depiction of female relationships in the past, lesbian certainly, but in general as well, I believe, and this is one of the cases I happen to agree with her. There need to be healthier depictions of female relationships, regardless of sexuality.
You are right that Briala has provided Celene good counsel in the past, but that's Celene's relationship with her spy. When discussing their actual relationship as people in love, it is couched repeatedly in the language of dependence on the part of Celene. She needs Briala to get adequate sleep. She needs that secret cup of tea to get started in the morning. She refuses to contemplate giving up her time with Briala for the good of the nation she rules, even when Briala outright tells her she won't have to, and that she needs to marry one of the suitors. Celene's relationship with Briala, the relationship itself, not their love, is presented as dependence and a weakness that directly allows Gaspard the support he needs to start a Civil War.