All of the arguments I've heard against using this method assume that you have adequate supplies for alternate rewarming methods. If you literally didn't, and it appeared they were going to freeze to death anyway, would you have a whole lot to lose? Of course, if you felt yourself starting to get hypothermic as well, then I'd guess you should stop and reassess the situation, but should you at least try this method first?
First, this answer is not a substitute for proper training. I recommend taking a class in Wilderness First Aid or higher to be better prepared for things like treating hypothermic people.
Second, hypothermia is a term that tends to have different connotations with different people. Sometimes what people call "hypothermia" is just a very cold individual, or they mean someone who is shivering uncontrollably for example. The stages of hypothermia are really a sliding scale with the varying degrees of severity sort of fading into each other. But regardless, treatment for 1st stage hypothermia is very different from severe hypothermia at the other end, especially in a backcountry scenario with limited or no resources.
For this question, a moderately severe hypothermic person seems to be the patient being referenced. Typical signs & symptoms would likely include:
Shivering has now ceased after previously shivering uncontrollably
If responsive, will likely claim to feel warm again despite their body temperature being dangerously low
Level of responsiveness is declining, entering into a stupor. Eventually becoming unresponsive or unconscious.
Not aware of much, if at all
Skin is pale and cold. Lips are blue/purple
Heart rate and respiratory rate are slowing to the point of being hard to detect
No longer able to warm themselves from their own metabolic processes
Needs immediate evacuation to medical care.
With this particular patient example, warming them in the field is not a realistic prospect. You need to seek help for them, and get them out of there ASAP. Body-to-body warming, if anything, is only going to help stabilize them and keep them from getting even colder. But even then it's not a very effective way to try to warm someone in a hypothermic state, especially one of any significant degree.
But in short, body-to-body contact could be used to treat this patient as a last resort, but only as a last-ditch effort to help stabilize them until help arrives, assuming help is on the way to your location and also assuming you have no other heat sources (fire, stove...). If no help is available then trying this treatment is probably better than nothing at all. Just be careful and don't make yourself in to a patient as well!
Side note: Don't let the person get this bad! Any treatment methods including the one being discussed here (which is not the standard of care) are going to be more effective the earlier you catch the hypothermia. Time is not your friend here, so act quickly.
This article from NOLS provides some relevant information to this treatment method:
According to Gordon Giesbrecht an adult cooled to 95°(34.7C), the common threshold for hypothermia definitions, can have a profound caloric deficit. ² A healthy adult at rest will make about 1 kcal of heat per kg of body weight per hour. This won’t be enough to quickly reverse significant hypothermia. In those tales where we got in a sleeping bag and heroically warmed the severely cold person – well - we may have done the patient a favor, but they probably were not very cold in the first place.
According to Dr Giesbrecht ³, the transfer of the energy to the core will be blunted by vasoconstriction. Since most of our mildly hypothermic patients are not dramatically vasoconstricted, this will be only a small hindrance. In severely hypothermic patients it may be a significant limitation to the heat transfer from donor to recipient.
People who are truly hypothermic are dangerously ill. They may be wasted (a non-medical term that says they are fatigued, dehydrated and low on food reserves), or have a serious simultaneous medical condition. We may not be able to warm these people in the backcountry. Our efforts are focused on stabilizing the patient; we don’t want to jostle them or allow them to cool further. They likely will only warm in the hospital, so we transport them gently.
I’ve been told two people in a sleeping bag to treat hypothermia is the “standard of care” in the wilderness. Actually, it’s not. The science isn’t strongly in it’s favor and in the wilderness, scenarios are often unique, our equipment less than ideal and our need to improvise real. As always, we’ll use our judgment, understand the principles of treatment and weight the factors in our specific scenario. The heat sources you have available on your wilderness trip may only be insulated hot water bottles, or the patient’s metabolism and shivering. If you have a good camp and plenty of people you may be able to spare a person to be in the sleeping bag. If you’re a small group, and knowing heat transfer between bodies is not very efficient, you may decide your assistant is best used staring a fire, making a warm meal and drink on the stove, setting up the camp or attending to the other people on your trip.
2. Giesbrecht GG, Sessler DI, Mekjavic IB, Schroeder M, Bristow GK. Treatment of mild immersion hypothermia by direct body-to-body contact. J Appl Physiol. 1994;76: 2373–2379
3. Hypothermia, Frostbite and other Cold Injuries. 2ed Giesbrecht GG and Wilkerson JA. The Mountaineers. 2007.
Sources: Wilderness First Aid from NOLS WMI, other training, and years of experience.
The article referenced above from NOLS.