Denis has a comment on Kafatek's website: "We also noticed that flat tamper base extracts slightly better than convex base. Again, we don't know why this is, its just something we observed. While convex base makes it easier to create better looking bottomless portafilter extractions, the extraction yield is about 0.2%-1% less than flat based tamper."
I think we all respect Denis well enough that we can take this comment as a great summary. Convex is easier to use; Flat, when mastered, can get slightly higher extraction.
Me calling it 'flavor of the month' is basically: While we can measure this, can we taste it? I understand why Flat's are more common now, especially with trying to extract as much as possible since this is a hobby for us. It's interesting,IMO, that La Marzocco ships convex tampers still with their machines.