MassiveD- You're asking the wrong person but I'll give you my understanding.

There are a few issues going on here. One is joint stability/alignment during brazing. It has been said by many that the better your miters are the less the after brazing alignment issues will be. The joint will tent to shift and if the miters are tight and consistent there will be no space for shifting to occur.

BITD of pressed lugs there was a large area of void just at the miter. So to insure the joint of max strength the filler would need to fill the void (and if you're using Silver we all know how it doesn't like this) OR to have the tubes touch and the filler flow between them.

Are the lugs designed to transfer the stresses of use through only them? If so then the tubes being cut to only enter the socket (and not butt against the joining tube) will be ok. I don't see this assumption being stated or suggested in any current lug manufactures info, suppliers listing, or any builder's processes. Granted this design might be the actual goal, but if so some one is doing a poor job of letting the building world know this. The exception to this is the use of a one piece head tube/lug socket that some high volume manufacturers used 20+ years ago. I think it's easy to say that the small builders were never intended to be in the loop as far as the engineering of this frame part goes.

Yes, there are possible millions of frames with poorly mitered, even square cut, tube joints out there. That the brazing and lug's strengths are good enough to handle this construction method is not a guaranty of absolute effectiveness. Just that the results meet the cost to produce VS warranty costs that each company sets for them selves.

I am not a good enough builder to say that my miters are "water tight" as some claim theirs to be. I have some faith in the strength of both lugs and fillets to fully handle the stresses of use. But i do strive for better fit up and miters and feel that this makes for a better bike. Whether you'll ever test out this betterness is not my concern. When you produce many frames that work you do tend to come to some conclusions about processes and their benefits. When you repair mass produced frames and see their failure modes you come to other conclusions. In the end you find there are many ways to skin the frame, some are more to your liking then others. Andy.