This is a great point—management really is its own skill. However, I think it’s also important that managers be skilled in the specialty that they oversee, especially if it is complex work like software development. I think that an excellent manager with no knowledge of software development (aside from attending an “agile summit” or something) would struggle to effectively manage a team of developers.

That’s what makes software development management so hard: to be good at it, you have to be a good manager and a decent developer, which are completely orthogonal skills.

At an early age, people are divided into "nerds" and "jocks", but many of the top athletes in school are also the top students. Similarly, management and technical skills are often developed by different people, but some people are good at both. The trouble is just that there is a very limited supply of those who are excellent at both and even so it's difficult to keep up both kinds of skills.

Promoting people who are good technically but don't turn out to be good managers is one pitfall, but another is not promoting people who are good technically, but would be more valuable as managers.

No, I don't want professional managers who are making decisions with no expertise in the actual business unit they are involved in. I have that problem at the moment.