Nick_Pyner:
The shield you allude to only provides a clock and an SD card, both of which you can do without, and does nothing to assist you in the actual collection of data.
I dunno, it depends on what type of data logging you want to do. If you are going to be disconnected from the host system for periods of time, I can image wanting to know the exact time an event occurs, and likely it will produce enough data that it would swamp the memory and EEPROM of the Arduino. I suspect that is why the majority of shields listed as data loggers have a real time clock and SD/micro-sd card. Not every data logger needs this, but I suspect a lot of them do.