I thought that accumulating metadata happened when you have multiple images selected and LR is looking over the selection to see what common metadata exists.
For example, if you select 100 images taken with a Canon 20D but with different lenses and on different dates, after the accumulation the metadata panel with say "Canon 20D" for camera but the lens and date fields will say something like "multiple entries".
In other words, accumulating just means it's looking for common entries.