The slipping attributed to bowlines isn't within the knot which would result in the know coming untied. Arbos have much more faith, and experience, with loading plain old bowlines than other rope workers. The only annecdotal stories that I've ever heard about bowlines coming untied were in lw load situations or someone didn't tie the bowline properly. that's not a bowlinf failure, that's pilot error.
The slipping is because the rope loop opens up and doesn't stay choked on the piece. Then the rope hangs and the piece falls. This leads to sever butt-pucker and a localized drop in ambient air pressure as the whole crew sucks in, hoping that they haven't used up all of their luck that day. In most cases, they have, and the out of controll piece embraces the most expensive piece of property in the area. I know, I had that happen before I started using marls.
A marl and half hitch are not interchangeable. The hlaf hitch stays snugged onto the piece and doesn't move around or droop like hip hop pants. A marl can slip off the butt end of a chunk with the same results as mentioned above. The only difference is that the pressure loss is less because the piece still should be held by the running bowline. The falling piece is only half out of control and will generally only crash into less expensive pieces of property.
Any time that a rigger thinks that a knot compromises the strength of a rigging setup it is time to stop and make one of two changes. Either cut a smaller piece or use bigger rigging. The loss of effeciency when using a knot, or hitch, should always be a non-factor.
Like others have said, using it as a butt hitch to reduce fall distance is uber important when working big wood. Pete Donzelli wrote an article about that titled "All in all, it's about reduing the length of the fall"