I'm using them more and more.
Upsides are:
1. They usually pay some tax, so long as they like you more than the pope. Not as much as doges, but decent money.
2. They don't expand outside their de jure borders.
3. They're always your culture.
4. They have a bunch of modifiers that make them very likely to like you more than your other vassals and so they don't rebel. If you crusade and get a lot of piety, they're rock solid.
I mostly run elective realms, so I make all dukes of my dynasty. If I don't have a good dynasty choice, I make a prince-archbishop. I also like to have a mix of prince-archbishops to add a mix of very stable vassals in every region in case things get rocky. They're also a great choice for very large duchies like Brunswick that you don't want to ever see unified with another big dynasty to create a nasty super-duke. If the area is same-culture, I might make a doge for the money. If it's not, prince-archbishops are great.
After an invasion, they can be really great. You can give all the churches in a duchy to one bishop so he has enough cash to upgrade his holdings. And he'll be a nice stable vassal in a new area. A mix of these guys with feudal dukes can be a great replacement regime.
Just don't over-do it. You won't get dynasty prestige from them. And as was said, the barons under them will be poor and won't upgrade their castles. Too many will hurt you over time.