When to use a double rope

Double Ropes?

Single Rope: Used in single form, as the name suggests, it is designed for rock climbing. It is best suited to relatively straight pitches, and routes from which the descent is not by abseil.
Double Rope: Double ropes are recommended for mountaineering, long rock routes and climbs where an abseil descent is necessary. They are equally reccommended when gear/runners are of dubious quality especially in snow and ice.

Advantages of Double Ropes

A set of double ropes normally consists of two "half ropes". These ropes were traditionally of 9mm diameter but recent advances in technology and manufacturing techniques mean that some full (single) ropes now have 9.4mm diameters and some half ropes are as small as 7.7mm in diameter. This reduction in size generally hasn't compromised strength in any way but there is improved handling and reduced weight to consider. The only down side is that a thinner rope may be more susceptible to breaking if loaded across a sharp edge.

Ropes are traditionally either 50m or 60m in length, the later being recommended for long mountain routes or for alpine bolt-clipping where abseils points are often exactly 50m apart!

Multi-Pitch rock routes

If the route takes a fairly direct line and the descent from the top is normally by foot then there is probably no advantage to using double ropes, however most multi-pitch routes tend to wander somewhat, either to gain, follow or avoid certain features - this can lead to rope drag.

Using a double rope it is much easier to avoid bad rope drag. Assuming good runners, each peice of gear is placed and then clipped into only one of the ropes. If your route takes you under, then around a roof then you can clip both (or one)rope into gear well below the obstacle, and one rope into gear at the obstacle (well extended, of course) and clip the other rope into gear above the obstacle. The effect of this (once you're above the roof) is that the second rope can be mre closely controlled (tighter) if required above the obstacle and the other can be looser to eliminate much of the drag. With a single rope, extra slack would have increased the distance in any fall.

Abseil Descent

If you need to abseil back down then you need to consider that a single rope permits only a 25m abseil. This is likely to be unsuitable in the mountains/alps. In the Alps, where abseil/bolted belays are maintained they tend to be almost exactly 50m apart so double 60m ropes are highly recommended. Although a 50m rope stretches under load you'll definitely be glad of the extra few meters!

Additionally, abseiling 50m in one go is probably twice as quick as abseiling 25m twice. This is because finding and securing a good belay/abseil stance for yourself and your partner is probably the most time consuming factor. And in poor weather (or an emergency) you'll want to descend as quicly as possible.

Poor Gear/Runners

In winter (or on iffy rock) there is always the concern that your runners might not hold if you fall on them. Using a double rope helps in two ways:

By placing two peices of gear at the same height (where possible) and clipping one rope into each runner a fall will load both runners, hopefully at the same time. The effect of this is to reduce the load that each runner must bear which in turn reduces the chance of the rock/ice failing.
By placing twice as many runners (even if they're not placed together) then you increase the chance that some will hold, and if they don't they will absorb at least some of the energy during your fall making the final impact (assuming everything fails!) at least a little better.

Self-Rescue

Perhaps something not often thought about at the local crag, self-rescue in the mountains is likely to be much easier if you are climbing on double ropes. Assuming the stranded/injured party is conscious/coherent then tie off and make sure they're secure. If they can untie from one rope you can abseil down to them, or, if they're stuck you can set up a hoist (which might not be nice for a half rope) but with two ropes your options are greatly increased.