Haven't heard an argument against using the parking brake except in climates where rust and/or freezing might be a factor. But I certainly live in such a climate, and have always used the parking brake without problem. In fact, not using it, and not regularly moving the parking brake cables is more likely to lead to rusting and seizing, so that when it is needed, it doesn't work.
When Park (gear) is used, the parking pawl and front drive train take up the load (tendency of the car to role, which would depend on the slope). They should be able to handle the normal weight of the car when on a steep slope. What might cause this not to be true is if the drive train components are subjected to shocks which can happen if the car is bumped while in Park, or the parking pawl is engaged while the car is still moving (ouch!). In this case, what would give way first is a good question. There's the CV joints in the front axles; the front differential side, spider, ring and pinion gears, as well as the spider gear pinion shaft; the pinion drive shaft, the reduction gears; and the parking pawl itself.
The first photo below shows the parking pawl -- it's more visible here with the upper reduction gear and tranny output shaft removed. Note the wide tab on the pawl that the arrow points to. That's what does the work. (This and the next photo are from the VTD version, but as far as the parking pawl goes, it's the same in the 4EAT with MPT, and the CVT.)
The second photo shows the tranny output shaft from the VTD version, with the reduction gear, differential planetary gear set, and the splined hub for the AWD clutch. (In this case the clutch is used to lock the planetary differential.) We also see a notched ring behind the reduction gear. It's these notches that the wide tab on the parking pawl engages. Doesn't look like much, but essentially the same thing is used on millions (billions?) of cars world-wide with rarely a problem, in normal use.
There's no doubt in my mind that using the parking brake whenever parked is far better. First off, it adds two more wheels on the ground that are not able to turn, so in the event the fronts are on a slippery surface, the rears might still prevent the car from rolling, and vice-versa. Moreover, with the parking brake applied, if the car is bumped, the brake adds another level of braking to the car, and could very well prevent the front drive train from being stressed, or worse still, damaged, by the bump. And then there's the case of parking on a slope and finding it difficult to take the car out of Park. If the parking brake is applied with the gear in Park but before releasing the foot brake, the parking brake (if working properly) will prevent the load from being applied to the front drive train. The parking pawl might take on some load if the car rolls a bit, but it won't tend to get jammed as it might without the parking brake.
In regard to not servicing the differential, the impact of good lubrication is, in my view, uncertain. All the gears (teeth) are under "extreme pressure", especially when accelerating, and so wear on their contacting surfaces is going to be exacerbated. But there's also bearings that depend on the gear oil, and the spider gears rotate on the pinion shaft without any bearings, so again, wear can be a factor. When the gear teeth wear and the contact force areas change, there is more likelihood of a gear tooth breaking off and this can then cause collateral damage. But I think predicting what would give way first would be difficult. It's these case where a minor original manufacturing variance that would normally not affect anything for the life of the car become more critical factors. These could be anywhere among the parts that depend on the gear oil.
(p.s. The reduction gears are lubed by the ATF and are not affected by the gear oil in the differential.)
subiesailor: as you have a CVT, I've added a pic of the pawl from it. It's actually better than in the first photo -- much more close up and in color. It shows just how rugged the pawl, and the notched ring behind the reduction gear, actually are.
The upper right also shows the multi-plate clutch -- the reddish ring is on one of the the clutch plates.
The new CVT uses a different design of reduction gear, but the purpose and location are essentially the same. The lower reduction gear drives the front differential pinion drive shaft, as in the earlier transmissions.
Last edited by plain OM; 10-05-2011 at 08:01 PM.
Reason: add p.s. and CVT pic