The "fine" in Diafine is a bit of a misnomer.
It produces fine grain relative to the exposure index with certain films.
For example, Tri-X in Diafine (my EI is 1200-1250) delivers grain comparable to Tri-X in Rodinal at the nominal speed. That's pretty good.
OTOH, TMX in Diafine gets only to EI 160 and is quite grainy, surprising for what is normally a virtually grainless film. And with TMY I get finer grain at EI 1600 in Microphen than from this film at EI 500 in Diafine.
So it all depends on the film.
It's a quirky developer whose chief benefit is convenience. It also produces a unique look that can be very appealing but, again, only with certain films.
For example, FP4+ at EI 250 in Diafine is a very useful combo for me, especially for nighttime photography.
Also, without Diafine I'd have had a difficult time doing any film processing at times this summer when the coolest my darkroom ever got was around 85F.