um I have a tendency to do this alot when drinking. See profile pic for example. Which for the record is an absolutely horrible idea. I have yet to hurt myself but I spent all summer climbing trees as kid so I have some good muscle memory. Also I'd like to reiterate I know that this is a bad idea. I've randomly hopped on a tree while out for a run or walking with my husband, camping ect... too. I wouldn't say I do this for fitness. Most trees aren't really big enough to do it a huge amount or for a long enough time to count as fitness. Plus if they were you'd be really high and risk hurting yourself, or the branches would be too high to get up into to begin with. I think if you want to climb for fitness you should try a rock wall. Plus they give you safety harnesses. I just think its fun to hop up in branches a every now and then.
I love climbing stuff, also especially when drunk. Trees are fun and challenging, but you can go much higher, much safer, if you find a suitable building!
not to be a downer, but I have a friend who was climbing drunk, slipped from a branch (granted she was jumping from the railing of a porch) fell and had to spend the next few months in a body cast!!
....but I must agree that I love climbing and did often as a kid...
I mean, what else are you going to do growing up in the woods!