I am still split and have saves with both choices.
Save 1: My Dalish Keeper would have no problem and also feels the obligation to drink herself. However, she knew by that point her LI Solas well enough to feel that he was very very uncomfortable with her drinking it. Together with her own magical knowledge about a geas, she decided to put her trust in her LI and pass this very, very tempting occasion to find out more about the ancient elves. She finds solace in the increasing indications (from Solas and other hints) that the ancient elves were not exactly the pinnacle of lawfulness and today´s elves should probably not really go back to the old ways, anyways. The same reasons apply for her to agree to remove the valasslin by Solas.
Save 2: My Dalish Keeper felt the need and obligation to use the unique occasion to access ancient elven lore. She also kept the valasslin. Knowing about her LI Solas feeling very uncomfortable, she is nevertheless a strong willed elf who can keep her own counsel. Solas in the end acknowledges her decision and sees wisdom in her, as not seen since the days of ancient elven, and also confirms her beauty despite with a valasslin or not.
So, both alternatives can be played with perfectly valid RP reasons which go beyond mere distrust or power hunger. But I be damned if I knew which version should be my cannon playthrough!
Only two things bother me: No discussion and explanation of that woman spirit from the well; and why is it the Inquisitor holding the Velluvian open and passing through as last, even if she did not drink? I am pretty sure the latter is a bug and hope the former is, too.