In case you haven’t noticed.
In later builds of 2.5x, if you switch the units to Imperial (in the scene tab) the grid changes to yards, feet, inches.
We americians thank you.
(All that dividing by 10 in the metric system is so confusing 
aljo
Like it or not, America has already gone metric in some ways (look at our industry), so you need to use the metric units if you’re modeling manufactured parts in Blender.
However nearly everything else is measured in Imperial units so I can see where that comes in handy to have in Blender, but even then I often refer to centimeters and millimeters for real small measurements because in those cases the inch is too big.
What AD, 64ths of an inch not precise enough for you? 
I think machinists in the US use 1000ths of an inch, why not use metric in this case I don’t know.
One thing I have used metric for is proportional scaling, converting decimals to fractions is a PITA.
Mentioning a measurement of 2 millimeters, for example, is just easier to say and smoother than 5/32nds of an inch when small measurements are concerned, it simply doesn’t sound as convoluted. Just to get a point across though, I know it’s likely not accurate.
When the grid is set to Imperial if you zoom in closer than an inch the grid will eventurally break down into mils (thousanths of an inch.) Or at least it looks like mils (I didn’t count them, I leave that to someone else
I think it would be nice to see a break down of 16ths and 64ths before mils.
I’m just to old to think in metrics.