I happen to have a bit of personal experience with some of the bikes mentioned and ABS. I’ve had a chance to ride all of the BMW’s including the F650GS, the 800GS, 100, 1150 and every year 1200GS and also the 950 / 990 KTM and the Triumph 800. I do not have enough time on the Yamaha to comment on that. All of the following comments refer to technical off-road riding; i.e. steep, loose downhills, fast washboard, etc.
The BMW’s fall into a few different camps. Some, like the 650 singles have a very basic ABS system. These basic systems work better with ABS off because they are basic, not much change from the original system released in the early 2000’s. The larger bikes have linked or integrated brakes that hydraulically connect the front and rear brake circuits. This inter-connection exists even if the ABS is off. How is that important? Well, when you apply the front brake 20% of the braking force is also applied to the rear brake circuit. The problem (?) is that if the ABS is on and the ABS kicks in it releases the hydraulic pressure in both brake circuits. So a skidding rear wheel also causes the front wheel to lose braking. The BMW ABS systems have gotten progressively better in handling off-road terrain as the 1200 line has evolved. I’ve heard that there are some new features coming with the integration of the traction control and stability systems on the new 2014 water-cooled GSA. Will probably have to wait till Feb to see what they are.
The Triumph ABS works very well off-road. To my knowledge the front and rear brakes are not linked. However from my experience, in challenging terrain the bike was still better with the ABS off. It was such a PITA to turn off that I eventually got to the point where I didn’t bother. That works fine as long as you keep your speed down. But going to fast on a steep descent could still lead to a moment of pucker.
The KTM was easier to turn off and frankly, I didn’t really try to see how good the ABS was. The bike is so much fun and capable that I was to busy grinning (with the ABS off)
As for turning off the ABS, no manufacturer is going to make it easy. It needs to be a very deliberate procedure requiring defined input from the rider. Why, because of liability. If it takes deliberate steps to turn off, it can’t be claimed it went off by accident and contributed to a crash. Most of the time (at least with the BMW’s) you can unplug a wheel sensor to disable the ABS. It will go into fault mode but will clear when the sensor is re-connected.
The real point of this is: know your bike, know your ability and be able to read the trail / terrain and adjust as needed. Which may include stopping to turn off your ABS. (Or just remove it as I have done to several of my bikes)
My $.02. YMMV. yada, yada, yada