I manage a group of engineers; I've spent about half of my career being an IC engineer and half managing engineers, and it's been intertwined -- in this company, I started off as an IC, then became a manager, moved to another group as an IC, then became a manager. When my boss proposed to me that I manage the group I manage today, I declined because I didn't think I was technically competent enough -- I'd never actually built the huge, scalable, systems they built, and I knew they could run laps around me.
Eventually, he persuaded me to take the position, with my team's consent. On my first day with my team I sat down with each person in the team and literally my first question to each of them was "What's my job around here?" And they told me they didn't need or want someone to review or approve their technical decisions -- when they had doubt, they talked with each other. They wanted someone to help them understand our customers a little better, and that's why they wanted me.
Generally speaking, I figure my job is to act as a retention aid (my presence around should make my engineers want to stick around more than if I wasn't around) and doing whatever the hell my team needs done that engineers don't want to do. I have technical opinions, sure, and sometimes I even disagree with my engineers. And they do whatever they think is the right thing to do. I think about 80% of the time we disagree, they're right.
I'm good at some things; I'm bad at others. I wonder if the issue is not whether or not a manager is technically competent, but whether or not a manager is competent in the area in which that manager actually spends their time, and their team expects them to spend their time.