So a couple of nights ago I was thinking about how Solas was so much against Cole becoming human.
I wonder if his motivations might go beyond the obvious (that he prefers spirits).
For starters, we know that Solas is not too pleased with the state of the people of the Thedas he has awakened to. Cole would, by necessity, become a human of that sort, rather than an elf still connected to the Fade in the same way as before--or at least, this is what Solas assumes. In essence, Solas might be worried that Cole is giving up his actual personhood for this mere shadow of life that exists now that the Veil is in place.
And then, Cole is not just any spirit. He's not something common, he's Compassion--which is evidently rare. I have my own theories (and they have some foundation in what we've seen actually happen) that spirits don't just reflect what's around them, they also help build the emotion/idea that they represent. So, if a spirit of compassion is lost, then Cole can no longer encourage people to be compassionate, making compassion rarer still.
Then there's another possibility, and it's something I think not many people have considered. In ancient times, spirits could take flesh, but it likely took a long time for them to come to that point--centuries, even, as their individuality began to manifest and they began to work through their issues rather than forget their problems to avoid corruption. They understood the waking world better, too, because they weren't separate from it. Thus, they were better prepared to face it without quite as much risk to themselves. While I don't know what negative effects Solas may be anticipating for Cole, it stands to reason that one of the reasons he's against Cole becoming human is that he simply doesn't think Cole is ready to join the waking world. He doesn't understand it well--he's only had a few short years to try. Although his body is maybe 20 years old in appearance, his mind is still effectively a young child's, and if he becomes flesh and blood, he'll be flung into it as the adult he appears to be rather than the child he actually is.
Now mind you, that's only my take on it. But I'd be interested to hear others' thoughts on the matter, particularly on my third point about Solas just not thinking Cole is properly prepared.