...plus, if you're careful, you get to use the wood for things. Obviously you can't burn it, but sequestered carbon remains just as sequestered if you build buildings out of it than if you left it in the forest --- frequently better, because preserved structural wood doesn't rot as quickly as a dead tree in the forest.

Of course, you still need a lot of trees. The internets say that a rough rule of thumb for the amount of carbon in a tree is about a tonne. Therefore, given current carbon emissions of about 10^10 tonnes a year you'd need to plant more trees than that --- every year. (And then wait multiple decades. This isn't a short term solution.)

As you say, another reason to plant them is for local benefit. You don't mention that trees also regulate the local microclimate; they tend to make rain, for example. When Cyprus was deforested, it got a lot drier. I knew someone who was working to try and reforest a lot of it with the explicit goal of trying to fix some of the water supply issues; I don't know if they got anywhere. And, of course, they're just nice to be around.

Recent research show that there are about 10^12 trees in the world.

http://www.npr.org/sections/goatsandsoda/2015/09/02/43691905...