It was the price Harry had to pay for his secrecy. He did not warn anyone for the coming storm so they where all collected easily. It also showed how far gone Maeve was, her redcap kidnapped Mac from accorded neutral territory.
But not out of character for cold days Harry, like so many things that happened in that book. Harry had the opportunity to warn everybody but he did not do so. They were taken unaware.Sarissa does not seem to lie like a mortal. I think everything she says is true in the fae sense.Harry became more paranoid, partly because of the mantle I think.
This is a Doylist argument, not a Watsonian. I'm saying JB could have done more believable things to get the others there, but there was no reason to take Mac other than "I suddenly don't trust anyone".
JB had Harry play the distrust card for a reason. Maybe it was just the simplest way to get the others there, but the ways I proposed make more sense. So the Doylist question is, did he choose that method just to have an excuse to take Mac? And why did he want to have Mac there? What relevance was his participation?
There's no reason to take him, other than to establish that Mab was nice to him, that he had a healing factor of some sort, and for us to see him on the island.
Mab being nice was interesting, but could have waited until SG, when she arrived in the pub with Harry for the meeting. The whole " flatterer" thing could have been the reveal, rather than the bullet on the island.
The healing factor could have been revealed in the pub in Cold Days, too. No need for him to traipse off to the island, when JB could have had him knocked out after the confrontation with Before. Harry or Thomas could have checked on him, and noted that he healed up.
The unique factor wasn't healing or seeing him with Mab. That makes the unique thing getting him on the island.
So how is Mac relevant to the island? That's the question.