With that said... Why was a thread from 12/25/13 exhumed. Lol.
Threads like this get resurrected often - people will search "What knives do Seals carry", and this is the first thing that comes up in many similar searches - once they hit the front page several people comment. I guess it's better that having a new Navy Seal knife thread every other day.
Spartan fixed blades, certain custom makers like Bill Coye, and many of Harsey's other designs (CRK or and even moreso the Boker one) are all popular among the military, although I don't know how many will put up the money for one. I have no evidence that any of the warrior knives from any maker have been especially popular with American military, or any reverse grip knife for that matter. It is popular with Israeli forces, some Russian forces, and some others though.
Soldiers are conditioned to carry their gear in. Basically, gear is separated into first line and second line gear. The first line gear is their fighting rig and weapons, and the second line gear is their pack and all the equipment they pack in. Essential equipment will be further divided, so they can separate it from their bigger pack in case they need to ditch it (maps are always kept on their person, and reconaissance and communication gear can be carried alone in their own pack). They really do cut the fat, but with communications, water, supplies, ammo, night vision, reconaissance gear, computers, medical equipment, and all the other mission essential gear, there is really no way to cut it down much further. If they get in a bad situation they may elect to ditch some of their gear, but you don't want to leave much of that stuff for the enemy to find. If a piece of equipment is very important, they'll bring two. If they don't bring it, they won't have it, and not having it could be the difference between success or failure, life or death, etc. They don't have to keep it on their backs at all time though - it can be stashed in a hide and retrieved when they exfiltrate. You really start to aclimate to carrying a load after a bit, and their gear is engineered to disperse the load and make it as easy as carrying 80lbs can be (it's good to have boots for ankle support too). It always sucks to carry heavy loads though, and I consider 20-30lbs heavy myself.
Ultimately, I think it's a mistake to think that the knives special forces use are any kind of benchmark or standard. Modern military likes to use knives that double as utility and fighting knives, which isn't that far removed from the civilian market. Knives like the SRK, SOG's value knives and Ka Bar will always be favorites because they are the cheapest quality fixed blades you can find, and there will always be some people that invest more for a knife they like, or that offers features they think they need.