Depends on the # of tables i'd say. If you are playing 3-4 sngs you might not need it. I used to play sngs and i didn't notice that much of an improvement with or w/o Poker Tracker, because alot of reads need to be based on the changing aspect of the game. But if you are playing a few tables then you'll probably need it to characterize the players.
In ring however, as soon as PT crashes (too often) i suddenly do not have an f'n clue what to do for even a single hand. I couldn't play ring for a second without PT, but i wouldn't be completely lost in sngs/mtts. You also don't have regs at sngs/mtts per se, so it isn't like your stats add up like in ring. And mtts shove you around tables alot, often before you can get meaningful stats.
It also kinda depends on your BR. If you have a BR of 200 it probably isn't worth it but if you have a BR of 2k then it is 1. less of a dent in your roll and 2. you are playing higher stakes so it will pay off better.
An HUD helps alot in either case, but i don't see it as essential in sngs/mtts, where i see it as 1000000% absolutely no exceptions essential in ring games. It's also really hard/impossible to analyze your play/plug leaks in sngs, since it changes everything depending on the blind level in the tournament (and sng stats are less cut and dry than ring).
