I think the problem with following the manufacturer's recommendation is that when the manual was written (25 years ago, in the case of my K), the oils were very different. In 1986, we were using SF oil. Now it is up to SM. Because oils have gotten better, that is also part of the reason they are getting thinner, intervals have gotten longer. I would venture if the K75 was built today, BMW would be recommending 10w30 or similar.
Be that as it may, some motorcycles get used very harshly. Not meaning they are run hard, but even if you only put 2000 miles on per year, if the bike sits and you only take short trips, there may be a lot of condensation in the oil.
I won't lose sleep if I go 5-6000 miles with good oil. Even more with a quality synthetic. But I'm always inclined to change the oil at least annually because it sits a bit during the winter. Like others have said, oil is pretty cheap insurance.