General: 25.4 vs. 31.8

Originally 31.8 was introduced with a lot of smaller tubes = less weight marketing (a la the first aluminum frames). Since the clamp bulge is larger, it should allow for thinner tubes and therefore lighter bars. That's pretty much all bullshit. Depending on the bar, there is little to no to negative weight savings by using 31.8.
Some people say a 31.8 bar is stiffer compared to the same bar at 26.0 or 25.4 and therefore transfers more ground noise. That makes enough sense, but in practice I've never been able to tell the difference. In theory the large clamp diameter (and often width) should allow for a better stem hold or grip or whatever. I've never had a properly tightened 26.0 or 25.4 slip, so that seems pretty unnecessary as well.
The only place 31.8 really makes an easily discernible (positive) difference is in carbon bars. Carbon bars don't hold up well in small thicknesses. With 31.8 it is relatively easy to build a good, strong carbon bar which isn't going to snap after a few rides.

As far as my personal preference goes, I like the fact that both mountain and road bars are made in 31.8. That kind of interchangeability is welcome. But I think they look silly as hell in almost all instances. My mountain bike was set up with 31.8 and it worked fine, but so did 25.4, and it looked better to boot.

Here is an exception: (and one of my favorite bikes on here)

I think it is easier to find bars you need if it was 25.4 rather than 31.8
you can find specific risers drops and bullhorns in that size easiest thats just me when I am lurking in bike shops

31.8 might be taking over road bars, but that is not much in the over all market for handlebars.

well you can do that but wouldnt it be just easier to get a 25.4 stem?

Of course. I could just keep what I have now, but I would appreciate the ability to switch between 25.4 risers or drops and 31.8 drops or risers without having to switch stems. Now that I think about it, it wouldn't be too practical considering I prefer different length stems for road and fixed riding.

And haha, yes it would be ugly but if I do it, I certainly will face the music and post pictures.

essentially, you should have different stems for different bar types if you worry about your bike fit.. i use 120mm for risers, 90mm for bullhorns, 110mm for drops.