I don’t think I am a great authority on this, having ridden an ATV only once, but in my opinion the experience doesn’t transfer over at all, in fact for me my motorcycle experience was counterproductive.
I was on vacation in South America this spring, we left before I could get my bike out of the garage from the winter, and I was absolutely jonesing to ride. We found ourselves at a resort that offered ATV tours and I jumped at the chance. They said the terrain was moderate, I’d disagree, but that’s neither here nor there. I flipped it once but decided to continue on, then got caught up on some rocks and jumped off with the ATV teetering, that was enough for me. The reason I think my motorcycle experience was counterproductive was that as I look back at what I was doing wrong on the ATV I’m pretty sure I was trying to countersteer and if I had never ridden a motorcycle I don’t think that I would have done that. The bottom line is that a street bike is controlled by very precise, subtle inputs; from what I could tell properly riding an ATV is more like wrestling an alligator, not much subtlety involved at all. Again, don’t necessarily take this as the gospel, as it is coming from someone who spent about an hour on an ATV. On the other hand any experience on a non enclosed vehicle has to be of use, just remember that the skill set is vastly different.
My take on ABS is that I didn’t want it on my first bike. I wanted to make sure I developed the skills necessary to brake in the traditional fashion. If I were to get another bike, especially if it were a sport tourer, it’s something I would certainly consider. I assume that the reason you don’t find ABS on smaller bikes is mostly cost, $1000 on a $9+K purchase doesn’t seem too bad, $1000 on a $5 – 6K purchase is about 20% of the cost of the bike on just one option.