How important is taking attendance? I guess the real question is what do you get out of it? Is it to find out if people aren't showing up for class? Seems that anything other then an RFID swipe card is going to a lot of extra work. If you've got 20-30 students that's a lot of tracking. If you've got less then that, wouldn't you get a feel for who's not attending?
Do some schools require a specific number of attended classes for a belt? I just don't know what the big payoff is after you collect all that attendance data (I'm not saying there isn't one, I just don't know what it is).
The dojo I where I have been for the last year takes attendance every session. Sensei takes the book home at the end of every month and compiles attendance records (from two dojo in the city).
Each month someone gets an award for the most practices attended. The national federation does gradings based on you having had enough hours of practice to be eligible to test - and this takes the guesswork out (and for those of us who don't record our own training sessions, it takes memory out of the deal, too).
Keeping attendance - the other side of planning - if you keep attendance and someone isn't advancing, you can look at the records, see if they're hardly ever training, and point at that as an issue - if you see that they're training lots and not advancing, you can question your teaching/training method, and look for ways to get through to that person - there are different learning styles (VARK), and the traditional "watch me do it and you imitate" doesn't work for everyone.