Couldn't running an exit node be a cover for other activity? One that provides a reasonable doubt as to whether it was the operator or some other actor who did something unsavory from an IP address?
I thought there was a classic statement from the Tor developers that you shouldn't do this, but the closest that I found on the site is the part about not running an exit node from home (as it might make law enforcement more interested in seizing your home computer).
also seems to imply that it might be useful to run a node to provide cover for your own traffic (though not an exit node in your home), but that it isn't known for sure how useful that is.
I think the core argument against your suggestion is (1) having your devices more likely to be seized is just plain harmful to you; (2) if you're personally doing something that law enforcement cares about, having your devices more likely to be seized increases you risk that they could discover that by seizing those devices; and (3) there may be traffic analysis techniques that law enforcement could use to distinguish between your own traffic and your exit traffic, like trying to correlate inbound Tor circuit activity with exit traffic, and attributing the traffic to you if it couldn't be matched up with an inbound circuit.
This is a bad idea because the police will break down your door based on IP.
It might be a good idea in a prosecution to raise reasonable doubt. Few people are willing to play punching bag for the police to find out. Also the general technical skill of the average cop and prosecutor is quite low.
