I guess, if you run an illegal marketplace/shop on TOR with your own bitcoind for payment (processing).
I guess features are not implemented to feature running illegal businesses.
You don't want this bitcoind on clearnet for privacy reasons/to keep your anonymity/don't want to risk to expose your real hosting IP, via Clearnet leakage by bitcoind.
Sure. That assumes, that bitcoind will make non-torified connections incoming or outgoing. Not sure about that, see below.
Unmodified bitcoind can connect through tor, but it can only have outgoing connections, not incoming ones.
Are you sure about that?
What kind of incoming connections are we talking about? There are two kinds of incoming connections. I haven't found a good summary and try to explain it. When one fetches a web page, you usually do not need any router port forwardings / open ports. Not an open port in that sense. Bitcoin is capable of the same. I've used Bitcoin-Qt inside Whonix, where no incoming connections of that sort are possible without using hidden services and it worked [tested client functionality only]. [In Whonix also no untorified connections are possible.] So in client mode, Bitcoin does not seem to require any more connectivity than a web browser.
Using a hidden service can be compared with the anonymous equivalent of opening a port in a router. Both, a hidden service and an open port allow one to run a server. Server software will be able to answer requests by other clients, such as web servers will be able to answer web browsers and they knew nothing about beforehand. and didn't expect [In comparison to pure client mode, clients such as web browsers can only get incoming connections for stuff they recently asked for and are prepared to receive.]
If it were the case, that Bitcoin makes non-torified connections when only using the socks proxy and not using Tor hidden services, then
https://github.com/bitcoin/bitcoin/blob/master/doc/tor.md should probably make that clear. At the moment that is not the case.
Also, they're a service to other Tor+bitcoin users,
I can imagine it's a service for the network.
have a peer they can connect to which isn't susceptible to exit node MitM.
If it's hosted by trusted peer, that would make sense. Otherwise risk to connect to the type of person "exit node mitm" would be even higher.