She is who she is because she is a follower of Shar and little to none of the conversations with her would make any sense if you could change her god and/or domain.
If you want to play a cleric and think there is only room for one (or the trickery domain just isn't for you), pick another companion. There are options, after all.
Well, "I Don't care" She is still how she is, even if the machanik is a little difrent. She is still Shadowheart. And I'm not asking to open all the domains for her, but some.
Look your argument is allready not valid (no ofence ment) sinze Larian let you PC choose a god freely and the domain aswell. For me this is not a nagativ it is a possitiv. I can still follow a god that I like, but I'm free to work with outher mechaniks and freedom is a plus.