Part of the bookkeeping is definitely familiarity. If you already know what all your spells do and have an idea of what you need, then deciding what spells you're going to prepare in the morning is easy, but it's almost certainly a daunting task the first time(s) you do it. Once you've done that, your bookkeeping for the rest of the day is a binary operation, either you have the spell or you don't.

For Psionics, it's reversed. You always have the same powers available, but you have to choose on the fly how much power to pump into them. It's actually kind of misleading to think of the flexibility as difficult bookkeeping, because, as mentioned earlier, for the most part, you'll either be hitting the minimum or maximum augments possible with your powers of choice. Yes, there are situations that may be better served by an in between choice, but for the most part, you can get away with the binary and still be decent. Familiarity with the system as a whole is obviously rewarded in pp efficiency, but the same can be said with magic and slot efficiency.

Since spells are more familiar to most people, it's easy to think of it as the way things have always been done, so the amount of choice you have when activating a psionic power seems more daunting when compared to spellcasting.

*Spellcasting in the above is obviously used to mean vancian spellcasting. Spont casters are a different matter.