I think you're supposed to get the sense that Roper is seduced by Pine and all his blushing around him; that there is something almost romantic or mythic in their relationship and this blinds him to the danger he represents. Logically it makes no sense but that only matters if you believe that Roper viewed Pine with bloodless logic. I think there's ample proof in the show, in fact it seems self-evident, that he saw in him something more subtle than just a surrogate for his business dealings and holdings. Whenever Pine demonstrates his charm in public or shows intelligence in negotiations Roper seems impressed, himself charmed and a little proud.
The show is really less about the machinations of plot and more about the nuances of their relationship and its dangerous intimacy. Even the threat Pine represents seems to only incense their dialogues. Is he Roper's closest friend, someone whom he can confide in after walling himself off his entire life and surrounding himself with flunkies and killers for company, or is he his adversary who's going to bring him to his end? That's really what elevates the show, makes it interesting to watch, if you commit to the idea that Roper, for the first time maybe, made a business decision based on emotion, and how that brought him down.

Too true, having the same man that you saw in the alps show up in Spain to save your son. Coincidence? Roper has been doing business for years and now, what he's blind?

I don't mean to speak for anyone else but myself, these are all just my personal experience, and conclusion at this time we all must trust our own experience.
Just because it didn't work for me, doesn't mean it won't for someone else.
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But who decides what is "forgivably unrealistic" and what isn't? Surely that lies in the eyes of the beholder

That's a multiple comparison fallacy.
Something can be unrealistic while still being forgivably so, because it makes up for it The Night Manager is unrealistic while not doing so, which makes the flaws all the more glaring.
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So.do you think Bond movies are more realistic, then?

Agreed. I watched it for Laurie, and I was sorry for him: what a mess to act in!