Ya gotta know the rules before you can break them. With a lot of things, there's "by the book", and then there's hacky modifiers that violate the book rules. You should know the difference before you break the rules. Rigging off your hitchtending pulley in a time of need isn't as big deal as rigging off your suspension pulley you use in the canopy. In the former case, if it fails in a climbing scenario, you have to tend your hitch yourself. Irritating, but no big deal. In the latter case, it could fail and drop you.
Same thing with rigging off your climblines. You might use your tail for some very light rigging, and it won't hurt to climb on it later. It's bad policy, and it's best not to do it at all, but sometimes you gotta do what you gotta do. In any case, "by the book" is usually the best way to go, and you should try to adhere to it.
For rigging off rigging pulleys, you can do what you want, minding the forces and capacities involved. It's always gonna be harder on a pulley than a block though, and the gear won't last as long. If you're getting close to capacity, you have to decide what the consequences of failure are. May or may not be a big deal.
