cinch,
Moss covered it nicely, but a few more points. I will often use a second tie in (sometimes the other end of the rope, sometimes another rope all together) for security and mobility. In wet or slippery trees the second tie in serves to triangulate you.
Many time going out a limb is the easy part! Getting back is often more challenging. Here a second tie in is a great benefit. Positioning a second tie in with a rope toss while in canopy is a skill you will want for may reasons. So in short in most cases when I use a second tie in it is temporary and not maintained for the whole climb unless needed.
Having said all that, my climbing style has evolved to include longer lanyards that are short mirrors of my primary system. This way I have the ability to secure with a second tie in by simply moving my lanyard from the side "d" rings to a frontal suspension point. For me this has proved the most versatile.
here the orange line is my lanyard, the green my climbing system
Moss's point about using a lanyard when not feeling comfortable is great. I would add, that as you are learning and developing your skills, form a habit early when it comes to lanyard use. The habit is this; as you are in canopy don't ask yourself if you should use your lanyard, ask yourself why you would not!
Tony