Tracy Tormé developed the Selar character when he wrote the script for The Schizoid Man, and intended to spread her out throughout the series and eventually develop her into a romantic relationship with Worf. But he left near the end of the second season, because of his feud with Maurice Hurley, and the romantic plotline with Selar was massively rewritten and developed into the character of K'Ehleyr.
They knew they needed a new doctor for Season 3 early on (Muldaur pretty much told them she was quitting around episode 7), but if Tormé stayed on, he may have brought back Selar, or prevented the introduction of K'Ehleyr as a replacement romance. "The Emissary" is near the end of Season 2, when Tormé (and Hurley) practically have their bags packed. Given the timing, Muldaur quitting in episode 7, Selar introduced in episode 15, I'm pretty sure it was a tryout for a replacement. The problem was that she was a Vulcan, and Roddenberry still didn't want to bring in a Vulcan character for fear that it would be too much of a rehash of TOS. Still, a Vulcan-Klingon relationship would've been something to see.