Yes they do; I have always heard it refered to as "Gaffing/Greasing out", and it isn't fun. If you are climbing in a tree with hooks, you need to be using a saddle and a lanyard. I have only been climbing for a few months now, but last week I had an experience in a 40 foot pine with a pair of hooks. These particular hooks have been filed more than a few times, (they aren't mine, I am still shopping for my own hooks) so the points on the spikes are more of a triangle with a sharp outside point. I won't be using these hoooks again, because they are just too worn out, and the angle of the point just isn't sufficient to get a good bite into the wood while standing in a comfortable posture while climbing.
With all of that said, I get back to the point; I was about 25 feet up in the tree when I was leaning in to move my lanyard up the tree to take another few steps up. I leaned in, and both of my hooks slipped out at the same time, leaving me hanging there hugging the tree while I was working to get my hooks back into the trunk. I was left a little bit bloody, and a little bit more cautious about inspecting the gear I am climbing with, before ascending the tree.
If you have nothing to use but a pair of hooks, I would leave the trees alone. In fact, I would say that you are better off with everything but a set of hooks, because if you can avoid using your hooks to climb, you should leave them on the ground. Hooks are hard on trees, and if you use them as a crutch when they aren't necessary they won't make you a better climber. I am shopping for my own climbing gear at the moment, so I know how much of an investment it can be, but on the bright side, the hooks are a very expensive part, so if you have those, the rest is downhill.
I'm a newbie to the tree work industry, so my advice should be taken as such. Any vetrans out there have any comments for us on this one?