Absolutely trees are beneficial. I don't think I'd ever live any place I couldn't go out and walk in them and smell them. They take in CO2, and expire O2 just like you say. Besides that they're a necessary crop for raw materials.
Some give off more VOC's than others. Evergreens give off a lot. A pretty good rule would be the more you can smell the tree the more it gives off. If you go out in the national forests on a hazy day, it is not the big city causing the smog.
I think what Reagan was trying to point out is that human beings are not the cause of all the world's ills. Some things are just beyond our capacity to control and we just have to live with it. Also, he was trying to say we can't regulate our way to a perfect world. EPA tends to think that with just a little more regulation, just a little lower limits, we can get to utopia. That'll never happen.
Yes, I think you're right on target there. Thanks for the comments.