I'm a forester so I'm probably biased, but the facts are that trees are very important to the planet. They are the second most important oxygen producing type of plant on the planet, they reduce global temperatures, hold soils in place and fix carbon dioxide into the biosphere. In tropical climates, like the ones you point out, trees have up to 90% of the biomass locked up in canopy and trunk. Unlike temperate climates like the northern US and Canada, which have more than 60% of their biomass in the soil and duff layers (though that percentage is changing due to non-native earthworm species that are chewing through our organic layer like wildfire). Many terrestrial animal species require forests for at least a part of their lifespan and the trees provide food and shelter to lots of critters. I could go on about the economic and biologic benefits of trees, but this would get to be a very long post.
As for planting them, a properly managed forest should reseed itself, but I'm a service forester (the person who comes out and looks at your forests when you need advice) and I will tell you the VAST majority of private forest land that I see is NOT properly managed. Way too many people fall prey to the logger who comes knocking at their door, offering them a few thousand for their trees with no forester involved, no management plan in place, no reserve trees, no no landowner protections, no security deposit and no recourse once the logger has whacked anything of value on your land and left you with nothing to even get seed from. Fully 90% of the house calls I make are to lands that already got the ax and the landowner is asking me what he should do now to get his beautiful forest back (uh, call me sooner?).
Even when you do know what you are doing, sometimes things happen and your regeneration fails. Having trees to plant and people willing to plant them helps keep those failures from being a long term ecologic desert.
It would be very nice if we cut down on fossil fuels, but there are very large and profitable companies that are working diligently for that not to happen. And on top of that, we pay them subsidies... *sigh* Until those companies squeeze every drop of fossil fuels out of the ground and every dime they can out of us in the process, the economic incentives just aren't going to be on the side of trees.
It would be easier if we treated our natural resources with the respect and reverence they deserve. Then maybe we wouldn't allow them to be abused. Unfortunately, people don't respect what they don't understand. They need to learn about natural resources and trees in order to gain that respect. One of the first ways a child learns about trees is to plant them.
I have seen this happen first hand. I plant thousands of seedlings every year with our local high schools. I present the 'Birth of a Tree' program to several hundred 3rd graders every winter. When kids plant trees, they learn a bit more respect for the world they are going to inherit.
So, yeah, it's worth planting trees.