Years of living and working with Johnny Storm would break the control freak out of anyone-- but there were still some things that Reed held onto more tightly than he probably should, but there was good reason for it.
If there was even one weak link in this chain, one person who might not be so close-lipped, if there was one person who could be a liability, the whole house of cards would fall and it would take down every one of them with it.
While Reed didn't exactly trust Doom and Luthor (or even T'Challa in certain instances), he knew where their priorities lie. He knew when and where and how they would move given certain stimuli. Really, it was one very large game of chess-- once you knew how the pieces moved, applying pressure here or there would result in a predictable reaction.
This mutant with the floating island though- who could speak into people's minds- he was an unknown, a variable-- and a powerful one at that. Untested, untried-- Reed had no way to predict what he would do from one minute to the next, let alone in a grand move such as this.
"No." He said quietly, shaking his head, "I don't know him-- do you? I'm not sure how he would react to the radical ideas that we'll be handling in this group. Additionally, if he's really from the future as he claims, his opinions may distort or entirely rupture the time line. Finally, I don't believe that he speaks for the majority of mutantkind, as opposed to one sect." A sect that could easily be/become a terrorist cell, a cult, any number of bad entities that were constantly being made due to one lunatic on a power trip and his followers; where as Doom was Latveria and T'Challa spoke for Wakanda. "I wish we could get Xavier." Reed mused, but dismissed the thought for now. There was nothing to be done on that front.