I'll take a stab at this, but I'm sure others can add to it.
Gears provide more stable timing than a chain. Chains flex and create harmonics that can create a slight alteration of valve events in relationship to the crank and ignition. These changes are so slight that most of us will never notice the difference, and unless you living life against a clock where these differences make the difference between going home and winning money, it may not necessarily be worth it. It may also be worth some piece of mind if you are running some close clearances and big cam specs to insure things never go bump at unintended times.
Also, a gear, while marginally heavier that than the toothed wheel, is not going to have a significant impact on the engines ability to accelerate. Again, these differences are so minor that you may not even be able to see them on a dyno and the gains in timing stability usually outweigh the additional rotational mass. They also do not directly add mass to the valve train as the lifters, push rods, rockers and valves are the primary areas where mass is the biggest concern.
There is no big benefit/gain to using them in a mild street engine. If you have them you may as well use them. They do have some value to others, so it may be a way to a make few dollars, but you won't get rich off them.
