Listening to the Gershwin album and LOS today. Seems to me, that by the time of the Gershwin album, Taylor's voice had become a very pleasing and important part of the vocal mix, integral to the sound of the Brian Wilson band. I miss that voice in the recordings and performances since that time. While Jeffrey is essential to the sound, the addition of Taylor gave the vocal blend a unique sound, beyond just recapturing some semblance of the patented Beach Boy vocal blend that the all male lineup produces.
Here's how I hear it. Brian's voice isn't what it was, of course... and he had a phenomenal voice of course in the 60's... so it always sounded to me, like they replaced Brian's beautiful voice with Jeff, and Taylor, because her female vocal mixed or doubled with Jeff's sounds similar to Brian's from the 60's. Then you TRIPLE it with Brian's voice, and it gives it the same accent and timbre as his original.
So she kind of 'sweetened' the blend and when doubled or mixed in the harmony I think it helps balance out Brian's rougher vocal. She's a little prettier (in both ways) than the BB's sounded but Brian's a little rougher so it all mixes out and helps make them sound so fucking authentic.
Then of course later as you were mentioning she was mixed more in her own space, or even given duets.
Either way, what's not to like? Always liked everything. EVERYTHING about her.