I'm not a real huge fan of frame handles, though I do recognize, respect and admire the extra skill and precision fit involved in their construction. Though some designs clearly mandate the use of a frame handle, often times it strikes me as complexity for the sake of complexity without any functional benefit.
And there are potential downsides.
1) A frame handle, like a full tang with scales, presents long stretches of metal / handle material seams where a small amount of shrinkage will result in a handle that is both unsightly
and uncomfortable. Yes, steps can be taken to minimize the effects of shrinkage - rounding the edges, fitting the handle material fractionally proud of the frame etc. But those risks can't be eliminated and simply don't exist (in the same way) with a full hidden tang.
2) Weight. That's a lot of extra metal in a handle with a frame design and this can contribute to making the knife handle-heavy. Less of an issue with a big bowie where you have a lot of blade to balance it out, but it can become a concern on a small, slender fighter or a hunter. I have picked up a number of frame handled hunters and found them to be much not to my liking both in overall mass and in its significantly handle-biased distribution.
3) Design limitations. With frame handles, some makers will make real efforts to introduce visually appealing and ergonomically sound contours to the scale material (John White comes to mind, as an example), but a great many frame handle knives - I'd say the majority - are pretty much rectangular in cross-section. Flat on top of the frame, flat on the bottom of the frame, with flat sided scales. Nothing wrong with that, just not my personal ergonomic preference - particularly on a knife designed for use.
4) Cost. More complicated for the maker to build, more costly for the customer to purchase.
As for the benefits, yes, it allows the maker to use scale material that would otherwise be unsiutable (in terms of dimension) for the knife in question. But again - personal preference here - I'd rather have the knife with a stag taper or roll or full tusk of ivory versus flat scales of the same material - particualrly on a large fixed blade. The knife below would not be improved, IMHO, with flat scales supported by a metal frame for the handle:
As for providing more opportunity for embellishment such as filework and engraving - yes, it undoubtedly accomplishes that. Though whether that results in a net benefit is often an open question.
I don't pretend to know all there is to know about frame handles, and I would be keen to understand if there is any functional benefit versus other construction methods. Does a frame make for a stronger handle, a more secure handle, a more comfortable handle? If so, I hope some makers will fill me in as to how and why.
I'm not saying frame handles universally suck - I've owned a few and will own more - on the right knife. But for me they are a choice that represents an exception to my rule of preference.
Roger
) even though the handle and blade were wider than the part where the guard is fitted. 
