Ah, my favourite psychopath from Mass Effect 2. There are a few things that would have made her story better.
First, I didn't want her to be disguised as her Mother. My only reason why I choosed her over Samara most of the time is that Morinth was more exciting, so I wanted more of that character.
Second, as mentioned, a good reason for picking her would be nice. As much as I wanted her around, I must admit that Shepard looks stupid when siding with Morinth. Having her valuable Info that would be lost otherwise or sth. like that would work I think. (My head canon is that she has the Information on the Shadow Broker instead of Cerberus. She always accompanied me on the mission to take him down).
And yes, the game really did not need more P/R Checks, but to be honest, I don't really care. In my last playthroughs of ME2 (long ago) I just added my character paragon / renegade points with the gibbed editor so I could play like I wanted to. If I replay the game I'll do it again.
In ME3, as mentioned, it would be most logical to see her in the monastary. I'd have liked to see the following choise. A couple of Ardat Yakshi wants to escape in a Shuttle. (They haven't killed anyone so far). Shepard has the choise to shoot them down or let them escape. If Morinth is there, she tries to prevent you from killing them (if you choose that) and you have to take her down. If Samara is there, it would be the other way around. You'd need to kill her to safe the AY if you want that.
Another thing no one mentioned so far. Citadel DLC. She was a squadmate allright, so why shouldn't she be there. I have this funny thought how she would be invited to the party and then mate with Javik only to find out that he in fact can survive her magic.
Well, somewhere I've read that her voice actor wasn't available for the third game, which is a shame (and Morinth without that seductive voice wouldn't be Morinth anymore). It would have been better though not to mention her at all instead of killing her off. Her fate would be left to your head canon then.