I can see that meeting happening - Shimano won't admit 6-bolt are better than centrelock. If SRAM decide to use centrelock is saying Shimano are better. Don't even get Campagnolo started!

This is the reason caliper brakes have barrel adjusters and QRs, discs (hydro or cable)would need the same quick easy adjustment.

I agree with mustard and can back up what he says from experience. A good wheel change can the difference between getting back onto the bunch or a ride back to HQ...

most of the various combinations of wheel and frame I own and have tried are interchangable with no issues

Yep :D

Although admittedly nothing to do with bicycles...

the point I was failing (and probably still will) to make is that generally the best way to deal with the inevitable problems caused by the stack up of tolerances across different parts and manufacturers, is to design adjustment/accommodation of on-the-piss-ness into any assembly...

And "Tighter Tolerances" means nothing without repeatability, you'd need to be sure that your zero adjustment required, highly accurate and toleranced assembly of brake and hub parts from Shimano/SRAM/Hope/Formula all bought and slotted together flawlessly in say 2013 will all still be directly interchangeable with parts from YM 2014/15/16-etc from every other manufacturer under the sun, far easier said than done...

An Engineering debate !, regarding hydraulic brakes.
Kinda re-inventing the wheel type thing.

Well, you're all Engineers then by profeesion.
You go to work everyday and earn your living by being an Engineer, right ?.
;-)

And for the record.
I have a road format bike.
I have fitted riser bars and hydraulic brakes.
It should be on the road this coming weekend.

Not many, how many bikes have to be approved by the UCI before they can be raced? ALL of them, and i seem to remember the UCI are going to enforce that all bikes must be available to the consumer. So whatever you buy has to be usable in a race situation.

Rims vary much more significantly in width and true also, so the issue isn't exactly a new one is it?

Do they? have a look at road wheels and there is very little variation. and the QR's that exist cope with what little that exists.

And being realistic if you could manage to get all manufacturers to hit +/-0.1mm from the face of the lock nut to the face of the disc mounting on each hub you'd be doing very well, then there's all the other parts to consider: mountings (even a close fit hole will add another 0.1 diametric clearance to the stack up of tolerances), caliper, pad spacings and the actual disc itself, all have an effect...

"Tight Tolerances" means almost unachievable in this context, hence current systems simply provide adjustments to account for this, a stack up of 0.3~0.4mm either side of the nominal position would not be particularly hard to imagine given all the components in the assembly...

And adding a facing operation to every bike would only serve to increase the potential for an even wider tolerance band problem on each bike, Facing only exists for either A) getting a surface nice and perpendicular to the wheels axle, or B) adding a tenner to the LBS bill for monkeying with a bike...

Having said all of that, Rims vary much more significantly in width and true also, so the issue isn't exactly a new one is it?

Wider pad spacing and lead in features might help but there will always be a variation in dimensions...