People say (for example here) that disc brakes are relatively heavy, giving this as a reason for not using them.
How much heavier than rim brakes are disc brakes? Is the extra heaviness in the brakes, or in the stronger rims, tires, and/or forks?
The difference in weight is there, but hasn't really been significant for the last few years. Scott, for instance, is building a complete, disc brake hardtail with a 4" travel fork. Bike weight: 7.45 Kg, or roughly 16 lbs. That's 2 lbs less than my road bike.
The weight comparison only becomes truly significant when you look at more basic bicycles, i.e. the $600-1500 market, because the cost penalties for premium parts, like truly light weight disc brakes and hubs, don't allow use in that market.
If you are looking at an aftermarket upgrade, expect to spend $1500 on new wheels and brakes, and often more, for quality, lightweight kit. The weight penalty has gotten down to only 50-100 grams in most cases, between a bike built with a high quality disc setup and a high quality V-brake setup.
In my opinion, the advantage in brake modulation and power, not to mention consistency in all weather conditions far outweighs the the addition of a few grams. No pun intended.
All of those examples you cite add to the weight.
However, I think the biggest complaint is the brakes themselves. Not only do you need a caliper which needs to be much stronger (and therefore heavier) because it's dealing with huge amounts of compression, you've got to have the extra disk hanging on your wheels, along with the associated mounting hardware for both parts.
I did a recent build with a surly cross check but was also checking the soma double cross with discs and the manufacturer specs said Avid Shorty Ulitimate cantis were 260g/pair vs the Avid BB7 mechanical disk rotors & pads/mounts at 1060g/pair. This is just those components and not the frame tabs or disc hubs which might also contribute small amounts to the disc set up. Those are among of the lightest cantis available and mechanical discs are heavier than top of the line hydraulic discs (eg Avid XX at 560g/pair) so the difference is not always that much. I don't know any hydraulics that run on drops so I went with the cantis and never had any no regrets. Even deep mud is not a problem, I just learned how to brake differently.
Disc brakes do add weight due to the caliber, pads, steel mounting hardware, and if they are fluid operated, then they are generally even heavier than the cable counterparts. You can reduce significant amounts of weight just by swapping the generic steel hardware for titanium bolts. Dollars per gram saved is quite unreal.
Lost in this thread is the weight savings on the circumference of the rim. With no need to have a brake surface, the rotational weight impact may be better with a disk even though the overall weight increases. I'll trade rim weight for hub weight.
Besides, how many gram counters (keep in mind that there are 28 grams in an ounce) can afford to take a dump before the ride and save lbs????