Based on this post and the one you have about the other bath in the house, the former owners certainly liked large sinks. :-) I love counter space, so that wouldn't have been the way I would have gone.

If you don't like the light fixture (regardless of whether it's "in" or "out"), replace it. It looks like they chose the mirror size based on the width of the light fixture, so after you find a new light fixture you can think some more about the mirror size. Plain, unframed mirrors aren't too expensive, so if you prefer full width, no reason not to go for it. I would install it so it butts up against the backsplash tile. The backsplash tile is definitely dated, but I have seen worse.

Is the light fixture dated

No, "dated" is a made-up concept to get you to spend money /replace something that works perfectly well. No, this light fixture seems to work well, gives plenty of light and isn't offensive in any way. If you're generally happy with it, I wouldn't touch the lights.

Also, is there any benefit to replacting the mirror with one that fills the entire space above the sink instead of the way this one has a wall bordering it on all sides?

No benefit. What you have is in good condition, and it gives you plenty of mirror space. Adding a few inches on each side wouldn't make any difference.

I love counter space, so that wouldn't have been the way I would have gone.

I was going to say the same thing. I hate this vanity because it provides zero counter space; it's obviously a problem because the OP has clutter lined up behind the duplicate sinks. To make a real difference in this bathroom (though this isn't as cheap as replacing a light fixture), I see two good options:

- Replace the vanity with a single-sink model that goes all the way to the wall /doesn't require cleaning in that bit of space between the vanity and the wall. This will also allow you a stack of drawers for each spouse.

- Replace the mirror with large, deep medicine cabinets, which will provide storage at the sink.

I appreciate the input!

I may decide to upgrade the mirror/light for a more streamlined look -- but that's simply aesthetic, because functionally the current setup is fine.

We don't mind the double sink vanity -- and we already have a large medicine cabinet on the other wall so no need to add another.

Thanks for you thoughts!