Personally, I would say this is a bad idea for pen-testers. As you probably already know, whoever is in control of the exit-node, or the last person in the chain of Tor routes, would be able to sniff that traffic as if it was originating from their network. Just because the Tor traffic is encrypted between nodes, doesn't mean it can encrypt the traffic to the final destination, unless it was encrypted in the first place.
That means whatever information discovered during a pen-test, which is supposed to help reveal security faults before the public can exploit it, would then have a chance of being disclosed to an anonymous person in the Tor community. And if you've signed a nondisclosure agreement, that would not be good for you if it was to be released to the public.
That's how I see it, anyway.
Put that in your pipe and grep it!