That's a good question. The hierarchical threshold multisig permission model used by Steem (and BitShares) is much more flexible and powerful than Bitcoin-style multisig (for example Hive account@xeroc mentions in a comment here that a member of a multisig authority is free to change their own keys at any time).

The problem is that our current permission types -- posting, active, and owner -- are too coarse-grained for third-party integrations. Finer-grained permissions seem like they might have some value, for example I can think of a few services off the top of my head:

• A vote management service like streemian.com can vote on your behalf, but not post.
• A post management service (which might e.g. mirror blog posts from your Wordpress site to your Steem account) can post on your behalf, but not vote.
• A trading console service (3rd party market UI) can place and cancel market orders on your behalf, but not transfer funds.
• A liquidity management service can manage vesting deposits / withdrawals and requests to move funds to/from savings to maintain certain level of liquid funds in your account, but cannot place market orders or transfer the funds to another account's control.

The management of third-party permissions from the UI perspective could probably be improved.

From a blockchain backend perspective, the blockchain isn't really designed with flexible permissions in mind. The internal blockchain API's, objects and the public protocol fields don't scale to M different possible permission types which may be delegated to N different third-party service providers.

It's going to take some design work to get this right.