We're not sure exactly why this matters. We guess in two-three years when your battery is pretty much dead, this could be a concernbut apparently, running your MacBook/Pro without the battery will cause it to run one third slower. Who knew? Who cares? Why are you running your Macbook/Pro without the battery in it? Is this a common occurrence out there that we don't know about?
Possible reasons for this include:
spikes in processor usage cause more power to be consumed than the AC adapter can provide causing power outages
someone at Apple is messing with the .0001% of the population that pulls their battery out of their laptop before using
