The reason some porro prism binoculars are less rugged than roofs usually has nothing to do with the split vs straight hinge design used. This doesn't mean that some split hinge porro binoculars don't have weak hinges, though. Some porros use a full length hinge along the pivot and STILL aren't as rugged as a well-constructed roof prism binocular. The reason SOME (not all) porros are less rugged than roofs is simple. Due to the dogleg shape of the porro prism binocular, there is frequently a lot of unsupported mass in the objective barrels both extended beyond and further away from the center hinge of the binocular. This creates a long lever arm that causes greater stress exerted on the hinge in the event the binocular is dropped on one of the objective barrels. In addition, the objective barrel of a porro is usually a separate piece threaded into the prism housing. A roof prism binocular has straight barrels, with each barrel usually of 1-piece construction, without the objective assembly being a separate piece from the prism section of the binocular. In addition, the hinge or hinges is usually wider on a roof than a porro of the same length, and a wider hinge resists being knocked out of colimmation better. This is not always the case, however, with some porro prism binoculars having wider prism housings and little to no objective tubes extending from the prism housing combined with a long hinge. A good example of this is the Steiner Military/Marine.
As for split vs. straight hinges, it's a mistake to assume one is always more rugged than the other just based on whether it has a split vs. straight hinge. Some split hinges actually have more bearing surface and are spread further apart along the pivot than some straight hinges. The strength of the hinge has to do with how well designed the hinge itself and the connection to the main tubes is. In some cases, 2 hinges spread further apart are actually stronger than a single hinge of the same total width with a narrower span. It depends on how much unsupported mass extends beyond the span of the hinge or hinges. I have no doubt that Chris's repair information with regards to split vs. solid hinge binoculars is true. However, I believe that has more to do with how well-designed the hinge is than whether or not it's split vs. 1-piece. Imagine a door installed with only 1 hinge, but the hinge is narrow vs. the same door with 2 hinges with greater bearing surface and those 2 hinges are spread further apart than the span of the single hinge. The latter is a stronger design, because it resists twisting about the pivot point much better. Some split bridge binoculars I've seen actually have wider total hinge bearing surface than some single hinge binoculars. Again, it all depends on the design of the individual binocular and the materials and manufacturing tolerances used. All else being equal, 2 hinges spread further apart with greater surface area than a single hinge will always be stronger.
The current popularity of the split bridge design has little do do with strength, though. It's popular because many people like the ergonomics/feel of split bridge binoculars and to many, it looks cool. Manufacturers will make what sells.
Edited by RifleDude - September/05/2008 at 15:58