Can you tell us the need for a chest strap?
Can you tell us the need for double D-rings on shoulder straps?
Can you tell us the advantage of having the harness dived into multiple pieces?
Can you tell us what happens when it comes time to replace the harness?
Can you tell us the advantage of buckles in the crotch strap?
Can you tell us the need for a soft "backplate"?
The chest strap helps to keep everything secure and minimize any movement of equipment, thereby allowing for fine, controlled movement in a variety of environments - particularly when in overhead environments. It also helps to minimize any protruding/dangling that may occur and maintain a streamlined profile.
The D-rings - all of them - allow for equipment to be attached wherever the diver feels it should be to maintain trim, permit maneuverability, and keep it where it may be reached easily. If the diver feels they do not serve a purpose, then he/she can easily remove them.
Having the harness divided into multiple pieces allows for full customization of the harness, easy replacement of worn straps (to address the next question on the list), and to made additions/omissions as the diver sees fit.
The quick release on the crotch strap allows the BCD to be put on and taken off easily, as well as for easy adjustment for comfort (if you know what I mean). If something gets caught or tightened a little too much, you can release it easily. If you are going to be in an environment where a release isn't desirable (or if you never find one desirable), then you can take it off easily enough and go with a solid strap. In certain situations, however, that release is more of an asset than anything else.
The soft backplate makes the BCD light, easy to transport, and allows to packing it up small enough to take along in a carry-on bag. I have a 14-year-old, 95-pound girl with a TravelPac (with a soft backplate) and for her it provides comfort, support, and a manageable weight. Don't like it? Swap it out for aluminum or steel.
Dive Rite harnesses, whether they TransPacs, TravelPacs, or VoyagerPacs, are secure, tough, and allow maximum customization so that they can be used in a wide variety of environments. They are made to hauled all over the world and into remote areas easily, and withstand brutal conditions. My instructor, a cave and wreck diving, trimix-breathing, master instructor with thousands of dives, is always diving his and swears by it. I love my TravelPac as does my girl. It's all about what you're going to use it for and what fits your needs.