Most people's reaction to Morrigan is rather indicative of the statement that reason is the slave of the passions, so let me be upfront about this and tell you first why I like her. The matter of trust will be easier to understand after that:
I like Morrigan because...
(1) ...she doesn't conform and lets no one and nothing impinge on her autonomy.
(2) ...she values knowledge and leaves no opportunity to acquire any secret knowledge she can find.
(3) ...she's a mage
(4) ...she dislikes the ruling ideology.
(5) ...she doesn't believe in the Maker.
(6) ...she's easygoing about sex.
Which means she has much in common with the type of character I would, most likely, be if I projected myself to Thedas as a mage. I understand her and she's a reliable factor. I know where to trust her and where not, because it's where I'd trust myself or not, and those characters I play who have a number of traits from the list above will know that their goals and Morrigan's are likely to be aligned beyond the current situation.
The dark ritual? My characters do it or reject it for various reasons depending on how I envision them, but *I* know how she thinks and I have no problem with her using sex as a tool towards an end I can understand. That means that even those of my characters who reject the DR do it without rancor or condemnation, because to do anything else would feel narrow-minded, and that's a trait I don't like to play.
As for accusing her of being a thief: to those who do that: may I ask if you have done the Crime Wave quest?
Also, everyone has goals they don't like to share with others. Why would I ever hold that against her? Everyone has secrets, too. So she was using the Warden to get what she wants - so what? Everyone tries to influence others in order to get what they want. Really, everyone, and it's not as if the Warden was any worse off for it. Quite the opposite, in fact, considering the DR.
In the end, trust is a matter of understanding and expectations. I do not expect my Warden's companions to tell me all their secrets and goals. I do not expect my Warden's companions to be perfectly honest. I do not expect that because I am not like that either, and neither are any of the characters I play, and neither is anyone I know. Also, trust, or its lack, is never absolute. There are always situations where you trust someone, and others where you don't. If you understand someone, you will know one from the other and the other person becomes a reliable factor.
I understand Morrigan and I like her. As for trusting her, the question is, as always: with what?