Disc brakes are already allowed under the UCI rules and regulation - have been since last year.
I can understand the want for these from the bottom up as weekend warriors look for the different things in a bike to someone that is racing - for cyclocross the introduction of disc brakes is a god send and it was allowed for a very good reason - it allows more people access to the sports as you no longer need two bikes frames if you want to race the road and the cyclocross season.
However on the road racing scene you just don't need that extra braking force - it's one of the reasons why in about 100 years the design of the brake caliper has changed very little - maybe you will see them in the spring classics when the weather is bad but really I highly doubt you'll see these in the TDF.
Remember when you have the whole road to navigate corners with, even in poor weather conditions you need very little braking force - the last rac I did in Gunma was a perfect example - pissing it down all day, carbon tubular rims (35mm) and Dura Ace Carbon brake blocks - coming into some of the corners at well over 40km/h with no problems with braking or maintaining the line.
The other question is purely racing specific "what advantage does this give a rider in a stage race?"
The answer is none what so ever, unlike off road racing where downhill sections are steep and technical raod racing they use public roads and the road surfaces are very good in comparison to thier off road cousins.
Actually I do beleive last year or the year before some teams switched thier riders to XC mtb for the descent of one of the mountains due to the course being very bad condition - will need to look that up.
But really on the downhills these rider are thinking about putting as much distance as they can between themselves and the peloton - so braking is the last thing they want to be doing.
The only advantage from a racing standpoint I can think of... By moving the braking surface off the rim, you remove some of the limitations road rims have had for years - thus allowing for more wheel innovation in rode biking. The change would allow for different materials to experiment with as well as profiles. Will something come from it? Who knows.
The extra weight at the hubs would be minimal from the ratating mass standpoint. The calipers are mounted low on the bike, so that minimized impact as well. The only issue weight wise I can think of would be the cabling/housing/mastercylinder.
---- From an enthusiast standpoint, I would love to have all my bikes running a 135 rear spacing. Having my wheels all interchangeable (and not worried about rim width) would be great for me. I would love having a bullet proof set of wheels for my rooad bike fitting my cross bike so if a friend visits I have two worthy bikes for a road ride. Also, would love to throw my cross wheels on my MTB for more efficient urban fun.
From an enthusiast standpoint, I would love to have all my bikes running a 135 rear spacing. Having my wheels all interchangeable (and not worried about rim width) would be great for me. I would love having a bullet proof set of wheels for my rooad bike fitting my cross bike so if a friend visits I have two worthy bikes for a road ride. Also, would love to throw my cross wheels on my MTB for more efficient urban fun.
This is exactly why the UCI passed that they could be used - it allows more people to access the sport - want to road race? No problem! Remove the disc brakes and run callipers, want to ride CX no problem run it with disc.
Cyclocross was seeing a dip in new junior riders and after research was done it was one of the main factors - running a strada corsa and a cyclocross bike was just too expensive for most juniors and thier family.