I design and research highway pavements for a living, and most of the arguments for driveway sealing sound like BS to me.
If you don't have a free-draining material underneath the asphalt, then that material will get wet no matter how well-sealed the asphalt is. Think of a tarp left on a lawn overnight - it gets wet underneath because it's a barier to the moisture coming out of the ground. An asphalt pavement works the same way. Water will get underneath a pavement whether it comes up from below or down from above - the critical thing is having a way to drain that moisture away.
Asphalt will oxidize and get hard from UV exposure leading to thermal cracking and poor bonding between the asphalt binder and aggregate. This problem is more severe on pavements that don't get much traffic. The reason for this is poorly understood. Many people think that the kneading action of tires slows down or reverses the oxidation process. The oxidized layer doesn't wear away as many sealant manufaturers claim, though.
On highways, asphalt pavements that become oxidized are usually treated with a thin overlay of asphalt concrete, or a surface treatment such as the "tar and chip" that was mentioned ("tar and chip" is a misnomer because in the US at least the binder is an asphalt emulsion, not tar) - other common surface treatments are fog seals (like chip seals but using sand instead of stone), slurry seals and micropaving. Generally, though, highway pavements run into other problems before they get heavily oxidized.
So, I think that the only advantage to sealing a driveway is to protect the asphalt from oxidation. I do not think that any of the sealing products actually reverse the oxidation process. Any sealant should be as good as any other, provided it blocks UV light, though some will undoubtably last longer than others.
As mentioned in a previous reply, spraying asphalt with diesel is not a good idea. Asphalt is refined from petroleum oil, and will disolve in other petrolem products. The diesel will evaporate back out, but in the process may break the bond between the asphalt and aggregate.
I don't know why one would use a coal tar emulsion instead of an asphalt emulsion for sealing. Both tar and asphalt are industrial waste products - asphalt is the gunk in the bottom of the still when all of the good stuff has been distilled out of crude oil. Tar is the gunk you get when coal (or other organic material) is destructively distilled into coke and coal gas. Tar is rarely used in highways in the US because it is very temperature suseptible - meaning that it gets sticky on hot days and brittle on cold days. Tar is cheaper than asphalt, which is probably why it's used on driveways. Both Tar and Asphalt are classified as "Bitumens", although Commonwealth English speakers will often use "Bitumen" and "Asphalt" interchangeably.
