Leadership Development & Coaching for Line Managers - Worth it?

Recently over at LinkedIn, I asked this question:

Just curious what people think about hiring a coach to do leadership development for line managers, supervisors, or high potential (hourly) workers.

Realizing that every individuals has different needs, what would be some of the general coaching topics that should be given (if any) for the above mentioned people.

What concerns, barriers, or road blocks would there be from:
a) the person selected to be coached
b) the manager of the person being coached
c) the peer group of the person being coached.

The intent of the question was whether the Management Team would invest in their Line Managers with an external resource.

Interestingly enough nearly most responses came back that it was a great idea to mentor and coach line managers.  There were several different opinions on who should be doing the coaching:

Internal versus External resources

Shouldn’t it be the responsibility of the Line Manager’ boss to mentor them?

In a perfect world, yes.

Unfortunately sometimes the Boss does not have the ability or maybe the time to do it.  So the Line Manager is left to figure it out by themselves.

They watch, they learn, they test.  And hopefully they make it.

What your thoughts about the positives and negatives of having a coach for a new Line Manager?