If you create an Impress presentation using one of the bundled templates, then insert a multi-row table, you'll notice the table cell background colors are striped - alternating uniform light gray and uniform dark gray colors. That may be appropriate for some themes, but not for a theme in which the background is white, the heading area is checkered-blue, and there's a hint of orange. The text is black or white, depending on the background, with no gray to be seen anywhere. In other words - the colors don't fit the template's theme/style.
This is true in 7.4, but not anymore in 7.5. Maybe due to Maxim's changes.
Indeed, the default has changed. The new default seems to be no-background.