Will admit it, I'd say I'm a bit of a minor knife addict and yeah some knives are bought for some of the cool factor/collection purpose than just strictly utility use on the trail.

As far as the big knives for backpacking, I'd say they're most useful for wood splitting as you can split faster and also thicker logs with a knife with a bit of size and heft. This would mostly come into play I think in wet conditions if you're needing to split bigger stuff to get to the dry wood. If it's dry, you probably can go with a smaller blade and be fine with firemaking.

Something of the 11 inch machete range, i would agree that's definately on the excessive side. My range of knives is pretty much a range of sizes and weights that I choose from depending on the trip. -Biggest/heaviest: Cold Steel recon scout (7.5" blade)
-Medium: kabar short (5.5" blade)
-small: cold steel roach belly or SAK
-even smaller: SOG Aegis, Leatherman Juice

I don't really see anything wrong with the slightly large variety of knives, Man vs. Wild guy uses a 5 inch blade I think? No doubt about it though there's knife enthusiasts such who'll consider buyin certain knives though just for fun anyways.

Cheers,