I've found it's actually quite hard to buy good mountain bike wheels that aren't set up to only work with disc brakes these days. And there's a bunch of fancy new designs for disc-brake specific road bikes out there. Disc brakes have buzz going for them. I can see why. Adding disc brakes to cars was a huge improvement because previous drum brakes had real problems with controlability and fade. But disc brakes on bikes pretty much mean that you move the braking surface from the rims to the hubs, both being equally "perfect" designs, so I tend to think of disc brakes as wasted weight.
Now, having the braking surface up on the hubs allows the brakes to work better when the rim is coated with water or ice. It also means that you don't need to worry nearly as much about overheating your rims while descending. On the other hand, people have been doing rainy descents for years and years without disc brakes without a huge rash of accidents.
In order to make a bike able to take disc brakes, you need to beef it up in all sorts of ways, some immediately evident, some not. The big thing to remember is that a rim brake is supported by the existing bike structure that needs to be beefed up in that region already. If you don't dish the front wheel inward to make room for the disc brake, it will be stronger than a front wheel built to an identical specification but dished. So, while some of the latest disc brakes made of exotic materials are fairly light, there's still an inherent weight penalty that is not offset by deleting the brake surface. And, for mid-end brakes the setup will weigh at least a half-pound more per brake set, likely more, just for the increased weight of the brake hardware.
One of the guys at the bike store pointed out that disc brakes will work when you've broken a spoke. I pointed out that I rode a 36 spoke wheel with four broken spokes and my V-brakes worked just fine against the wheel. Furthermore, V-brakes are also easy to service while on the road using only a simple multi-tool. Much easier than mechanical disc brakes and without the requirement of carrying hydraulic servicing equipment that hydraulic brakes require.
The thing that drives me crazy is excluding even the possibility of doing things any other way. Forks are losing the mounts for V-brakes. And I'm seeing several designs for bike frames that might be kind of nice, except that there's absolutely no way to not have disc brakes.
Most folks don't ride in conditions where a disc brake is said to be better. I do and haven't felt like my existing brakes were unsafe. I'd much rather spend the money and weight that disc brakes require elsewhere. Like on wheels that are strong enough to handle abuse. Or on safety gear.
I wrote earlier about the difference between being a weight weenie and reducing one's wasted weight. And I see the extra half-pound per wheel as wasted weight. The way I see things, if I were to decide that my v-brakes weren't suitable for my riding anymore, I'd most likely put a disc brake in front, potentially pairing it with a fork designed with a through-axle given that the fork manufacturers have finally realized that it's necessary to provide adequate rigidity and prevent wheel ejection. But, given that there's a very hard limit in bicycle dynamics to how much braking power a rear brake can offer... and also how the rear wheel and rim is naturally protected from spray and grime... I can't see any reason why I'd ever want to use a rear disc brake.
Given the troubles I've gone through trying to find replacement parts to my specifications, I'm just worried that I'll be forced to waste weight one of these days.