How often do you use a laptop atop your lap? When you do, how often do
you have the laptop tethered to a cord and brick for A/C power? More
likely the OP has the laptop atop a table or desk when on A/C power.
The "battery cover" that I've seen for the Dells was for the CMOS
coin-cell battery. Got an example of a battery cover for the main
battery's compartment? If so, the ad for it might explain its use.
There are venting holes in the battery compartment? The OP gave a D410
as an example. In the "Before Working Inside Your Computer" section of
its manual (link given below), I don't see venting holes.
http://support.dell.com/support/edocs/systems/latD410/en/ug/r_r.htm
I'm thinking the battery has the rubber feet so the absence of the
battery means there are no rear feet to stabilize the laptop and less
rubber feet means more likely to move during use (if on a table or desk
versus on a lap). However, without the rubber feet on the battery pack,
that means the rear of the laptop is closer to the table or desk. That
means more restriction in the air flow out the bottom fan air vent in
the case. Well, that was a concern when I was looking at the D410 that
the OP gave as an example. The service manual (linked below) shows the
rubber feet are not on the battery pack for the Vostro 1520:
http://support.dell.com/support/edocs/systems/vos1520/en/SM/td_batt.htm
I don't see venting holes in that battery compartment, either, although
it's possible there are small holes in the case that could allow air to
get sucked in. The pic isn't good enough to tell. If there are holes
in the battery compartment to let in air, and even if that shows a lower
temperature on the CPU, what about the temperature for the rest of the
components since air flow has changed?