Re: 2-piece blanks
Ferrules are certainly a weak spot, but the amount of thread and epoxy is negligible insofar as weight is concerned.
As far as flex and taper design, perhaps early versions of 4 piece rods were insufficient, but modern rods (if made be a reputable manufacturer) always ALWAYS compensate for this, and I don't think you or I or any human will feel a difference in a blind test.
Here's the catch for me: 3 connection points vs. 1 mean 2 more opportunities for something to go wrong. Maybe that third ferrule was wrapped too tightly. Maybe one of the 3 connections came a little loose during travel to your next river access and you didn't notice - that nice tree you just hooked turned your 4 piece into a 5 piece. Or maybe that blasted tip piece is misaligned and you just can't reach it to turn it straight without setting your rod down....
I still own all 4 pieces though... It's amazing how small that 4 piece 7'6" packs.
Tight lines and tight ferrules.