Except temporary companions have crappy builds. You get them handed to you in a set state and depending on where they are, that could just be more of a pain. When you recruit someone you use them for that quest like you did with wynne. Then you build her after that, but her build had good utility as she was a healer. While not so helpful with killing enemies, she could keep your team and herself alive or better their chances. Alistair would be a sheild warrior I guess. He could be okay but I suspect he'd be more dialogue than a companion. Hopefully if they made morrigan a companion she would start better in this game than the useless stuff she came with in DAO. My god, I never found anything she had to start even remotely useful.
That all just requires competent balancing of temporary companions. As well there is nothing prohibiting BioWare from allowing us to have 5 companions at once in those instances either, which would eliminate the problem many people having with being saddled with a character or class they may not want over one they do. There are a multitude of ways they could handle guests like this if they so choose to.
One way would be to let you choose their build when you get them. If you're level 20 then so are they, but all their stat and ability points are unused. Another way would be to have their questline they are involved with cater to their build and their build spec'd to their questline, so they are purpose built for those enemies and challenges. Likely decreasing the chances they'll be a burden, if not making them an asset. They could also have them be slightly over leveled in that case as well, so any incompatibility with your preferred party would be offset by their being slightly more powerful than you or the enemies.