Brambles are a really good food for rabbits. Not so good for the person picking them. though. I tend to cut the leaves off with secateurs straight into something robust (eg a box) and forget about the stems as I don't like shredded fingers. Some leaves are thornier than others, so it pays to look around - unless you are cutting them back anyway. They also dry quite well for feeding in Winter, although it's one of the few greens that you can pick all year round.