Basically it's doing one thing that Sui could never do - taking permenants off the board
This was certainly a weakness of suicide, but most people who had success with the deck, up until its last gasp, compensated with contagion, nevinryal's disk, or splashed green for pernicious deed.
The real reason why suicide isn't competitive is because it is forced into topdeck mode very quickly, and its topdecks are infrequent, because of little card draw, and underpowered compared with the rest of the field.
I test with a die-hard sui fan (who no longer plays it competitively), and although Night's Whispers helped some, the deck just can't recover in the midgame after the early blows have been traded. While black still possesses excellent metagame answers and great disruption, it's this achilles heel that keeps it a deck of the past.
The only current deck that I've seen that approaches what suicide was, is Akuma's (and Jaco's) B/R aggro workshop deck from GenCon. While it ran a piddly amount of draw spells, it was able to compensate with welder, which can turn dead mana accelerants and disruption back into threats that are relevant to the game state. That, and a first turn 5/3 is better than a first turn 2/2, regardless of its pestilence ability.
