Photocell sensors for light sockets, is it really worth it?

If the switch that controls this particular light socket is not connected to any other device that is drawing power, then it will draw only a trivial amount of current. Nothing to be concerned with.

What concerns me about this description you post, is that it is recommending it used for indoors.
In an indoor environment, other lights could be on, which will be sensed by this socket, causing it to turn off.

I suppose if you use one in an isolated location without any other lights, and can rely on window day light to trigger the sensor I suppose it can be used indoors.

What these really are good for are like the front porch where it is exposed to the day time light and switch the bulb off reliably.

Well it will, to the fact that you will never need to forget to turn it off and let it burning all day.

If u REALLY want it to save energy though, find one that can take a compact fluorescent.

And/or: Does it need to be on at all time at dusk? There is no other lightings in the surrounding? U maybe able to find a proximity sensor instead, so ONLY when people approaching does it turn on. That will save u even more.

This conversation is going nowhere until u disclose what's the intended application.