I won't believe the Seeker process ends at being "touched", especially since no actual clarification has been given on how it works. Being temporarily inhabited by a spirit cured Pharamond of Tranquility, yes, but as far as we know it didn't give him permanent magical abilities. Now, what are the examples, that we know of, where such powers are obtained via interaction with spirits (most importantly, immunity to possession). Wynne and Anders.
A spirit touching a Tranquil to reverse that condition seems to be all about reopening that person's connection to the Fade. It could be possible that Pharamond didn't gain any new permanent magical abilities because he was already a mage, thus already having a wide range of magically abilities because of his deep connection to the Fade, beforehand. So renewing his connection to the Fade only brought him back to an emotionally unstable square one on the magical scale.
Seekers, and I guess to a lesser extant Spirit Warriors, don't have the same deep connection to the Fade as mages do. So when a spirit touches them to either reopen and/or increase their connection to the Fade it possibly opens up the use of magical abilities for them.
Also note that Lucius said that "we are abominations, Cassandra". This is a word with very specific connotations in Thedas, and I don't think his use of it just happened to be a coincidence.
I'd take anything a madman working with a doomsday cult says with a grain of salt, but the full quote was...
Cassandra: So you did all this because you hate our order?
Lucius: We Seekers are abominations, Cassandra. We created a decaying world, and fought to preserve it even as it crumbled. We had to be stopped.
It's possible that he meant the spirit possessed type of abomination, but that gets a bit vague in context with his whole statement. He isn't talking about any sort of possession when saying that to Cassandra. He was talking about how the Seekers were wrong to protect a decaying world that they themselves helped to create.
Plus, if the Cassandra and the Seekers had something like a spirit of Faith secretly possessing them and giving them magical powers, I'm pretty sure people like Solas and Cole would be able to tell right off the bat and possible said something about it. It would be kinda hard NOT to discuss the fact that someone who is very distrustful of spirits secretly has had a spirit residing in her this entire time.
The closest conversation on the subject is between Cassandra and Cole, but that doesn't really hint at possession either...
◾ Cole: Oh. That makes more sense.
◾ Cassandra: Why do you look at me when you say that?
◾ Cole: You found faith, not just a feeling. It was a spirit.
◾ Cassandra: We do not need to speak of this further.
◾ Cole: I'm a spirit that touched a body, you're a body that touched a spirit. We're the same but backwards!
◾ Cassandra: Please, stop.
Cole gained human attributes when he came in contact with human Cole. It possible that mortals gain spirit like attributes when they come into contact with spirits.