What do you think is the Don's reason for appointing Tom Hagan consigliere?

It seems reasonable enough to have used Genco in the role - a man as experienced and bound up in the family as the Don himself, but to me it seems as if Tom is fairly useless as anything other than a cool-headed representative. Whenever the Don seeks Tom's advice it just feels as if he's testing him. You never get the feeling Tom could tell Vito something he didn't already know or hadn't already considered.

With a role as genuine adviser to the Don ruled out, is Tom made consigliere simply because Sonny cannot be trusted to represent the Don in sensitive matters? But surely it can't be that, because wouldn't it have been obvious to the Don that a man who cannot even be trusted as a representative, is inevitably going to make a hash of being boss? If the Don didn't think Sonny capable, surely sense would suggest Clemenza or Tessio should have been in line before him? If Michael hadn't come good, the Don's sentimental decision to make his oldest son his heir would have destroyed the whole Family.

Or perhaps Tom is simply appointed to be groomed for the post-Vito era? Not knowing he was going to be put out of action by the shooting, perhaps the Don didn't feel an experienced man like himself needed a match-ready, 'wartime consigliere' in the foreseeable future. Perhaps he instead wanted to fill somebody who'll actually listen, with as much information as possible to assist the hot-headed, irrational man he had chosen to make his heir? Again though, surely the Don would have been aware that Sonny wasn't going to spend much time listening to Tom? Again, bumping up the older, more experienced Clemenza or Tessio to consigliere would have been the sensible option. If Sonny was going to be 'counselled' by anyone, surely it could only be them?

I just can't really work out what the Don was thinking in giving Tom the job. Remove Michael from the situation and it almost looks suicidal. The Don would have appointed an unfit boss and a basically ineffective, non-Sicillian consigliere, while managing to overlook two very experienced capos. In my opinion, for what its worth (not much I warrant), Tessio, not Tom, should have been bumped up to consigliere, on the promise of the top job when the Don dies. Sonny should never have progressed above Capo, and Tom should have been left to a civilian career like the Don planned for Michael.

I know Puzo suggests the Don comes to realise that Tom's appointment was a mistake, but I'm just wondering if anyone has any theories as to what the Don was thinking when he made the move.