The uShield dock you pay only once and it is for people that is going to use several i2c devices in an Arduino friendly format, stackable. If you are going to plug just an accelerometer, then you can do it directly... except for the LIS302DL... it is a 2.5V device (actually 2.16 to 3.6), so you will need the i2c bus adapter board anyways.
It is strange, but the sparkfun site doesn't mention that you cannot use the LIS302DL with 5V... maybe they got too business oriented and forgot about engineering? 99% of MCU projects out there are still 5V, right?
Actually, now I'm wondering, what does the nunchuck do about it? Since people plug them right in, don' they?