Disc brakes on road bikes – a good idea?
There are a lot of concerns about disc brakes on road bikes. So if you are considering a road disc frame or road disc wheels, here is a frank assessment of the situation as I see it.  Information on disc brake hubs can be found here.

Power
People have raised concerns that disc brakes are not powerful enough for road bikes. Or that they are not good enough at dealing with heat, as they have to handle, in a lot of cases, a lot of heat in a short period of time. Downhill racing requries a lot of its brakes, however they have massive brakes and massive rotors which can deal with this – on a road bike, weight is a bigger concern, so can the brakes really handle it.
In response to this I will say the following: leave the design to the experts. Cycling always demands light components and that always presents challenges, to which a lot of innovative designs come through to save weight and retain performance. However as a braking solution, the disc brake is better. That is why they are used to stop aeroplanes, trains and lorries. Each of those requires much more powerful brakes than a bicycle and has to deal with a lot more heat.
In addition to this, consider how poor at dealing with heat carbon fibre generally is. It makes sense to out-source the braking to a dedicated brake, rather than simply endlessly wearing away at your valuable and structurally critical rims until they are worn out, making them weaker each time you brake.

Why aren’t disc brakes used on the pro-tour circuit?
Well, they arrived in in 2016. The reason for the hesitation is three fold. Firstly, teams are sponsored by manufacturers, not all of which previously made disc brake road bikes. So it would put them at a disadvantage if the launch of disc brakes were done suddenly. Secondly, the industry has not done a good job at agreeing on standards generally. However on road bikes, it is fairly straightforward, nearly always 100x130mm quick release, Campagnolo, shimano or sram. You can get neutral support from some providers like Mavic without them having to carry a huge variety of wheels. However, with disc brakes there is now also rotor size and axle type to think about, which adds enough variables to create quite a headache. Finally and perhaps the most pressing concern is that disc brakes work better, especially in the wet. So if you have a rider descending a major pass in the wet and is using disc brakes, they brake and their brakes work. If there is a rider behind them on slippery, wet carbon rims and rim brake; they throws on their brakes – they will not work as well, so a crash is probable.

Update: there has now been an acceptance of a 160mm rotor size standard in professional racing.

Weight
Well, it is pretty inevitable that disc brakes make for a heavier system. The rims may be slightly lighter, there will probably be more spokes, especially at the front, the hubs will be heavier and you have a rotor plus more cabling. That said, making bikes light enough will not be too much of a challenge, the weight saving will probably just reside elsewhere.

Is braking performance the only benefit?
As I said before, carbon fibre does not make a brilliant braking surface. It holds on to heat too much and gets slippery in the wet. However, it is a good rim making material, especially for aero tubs which is what the professionals favour. Carbon fibre naturally makes for tough spoke beds. So disc brakes and carbon fibre rims go much better together than rim brakes and carbon rims ever did.

What about rotor choice and axle type
This is always a compromise. I have an article on rotor and axle type if you want to read in more detail. However the bottom line is this:
You have to have shimano centre lock rotors if you want a shimano system in 140mm. 140mm offers poorer heat dissipation, less power and less modulation, however it is lighter. So for my money, I would go 160mm rotors if you have the option. That gives you the full range of hubs and rotors so you can pick whatever you want. If your frame/fork can only take 140mm, then you have a choice, shimano centre lock rotors which requires a centre lock hub. Shimano brakes with rotors from another manufacturer – TRP, braking, avid etc. Or brakes and rotors from another manufacturer. A lot of frame/fork manufactures have gone down the 140mm route though to make for a lighter system, partly the lighter rotors but also because less of a strain is put on the frame/fork so they can be lighter too.
In terms of axle type, well, in my opinion I would go through axle. It is better system for mounting a wheel to a frame. The downside is generally weight. The upshot is increased strength and stiffness.  I would also come down in favour of 15mm rather than 12mm as it is stiffer and stronger and shouldn’t add much weight, however even 12mm will offer improvements in stiffness over quick release.

Are they necessary?
Well, it depends. If you are pushing the boundaries of performance. I would say yes. You can get a stiffer, stronger bike with better braking performance. If you are on a budget, it may well mean you get a heavier bike with brakes that do not necessarily work any better, especially if you have cheap discs with 140mm rotors. If you only ride on your own and only ride in the dry, it may well just not be necessary. I think the key question to ask yourself is; how limiting do you find your brakes to be at the moment? One additional key question to ask yourself is how much do you want to spend on your wheelset? In the past, wheelsets were much more disposable and often can be fairly cheap and light. With disc brakes, they are likely to last a lot longer and cheaper versions are likely to be a lot heavier, so there is a compelling case for investing in some more durable, more serviceable hubs and perhaps the use of more exotic materials to get the performance that you are used to from wheels back.

If you want to learn more about axle interfaces, you can find information here.  If you want to see some road disc brake rim options, you can view that here.  If you want to look through disc brake hub options, you can find that here.