The non-DOHC boxer motors have 6K valve checks with screw/locknut adjustable tappets. No parts/shims and a valve check/adjustment can be done in less than 30-minutes, half that if you're practiced. And you don't have to have a shim kit or have the downtime of swapping/ordering shims at the dealer.
The latest DOHC boxer motors are shim, but the heads remain extremely accessible and there is no need to pull and re-time cams to change shims. There is an initial 6K check, then ever 12K.
The S10 is 24K, but getting to the heads is a fair amount of work and if you need to change a shim, you have to pull and re-time cams which is no picnic and a huge opportunity to do some very expensive damage to your bike if you don't get it right.
Personally, I don't think there is any moral high ground where one of the approaches to clearly superior to the other. The earlier boxers have frequent checks, but they are dead simple. The S10 checks are the least often, but a royal PITA. The new boxer is somewhere in-between.
The idea that longer valve adjustment intervals reflect a mfg's general technical superiority is BS. An air-cooled engine is fundamentally different than a water-cooled one and will have different maintenance requirements and intervals. And valve adjustment intervals, like oil change intervals, are pretty arbitrary anyway, representing a judgment call for the best compromise between service costs and reliability. Different mfgs make different judgments - I think Suzuki and Honda are at 15K or so right now for most of their bikes and I doubt this reflects anything more than being a little more conservative and/or wanting to help dealers out with a little more service work. I wouldn't read any more into it that that.
In general, I think BMW's do have somewhat higher services costs, but not because they are harder to service. They're simply a more upscale product tending to be sold at more upscaled dealers to people who can afford to pay a little more. For example, most BMW dealers provide service loaners to customers, but this is almost never done in a Yamaha dealership. It costs more to service a Lexus vs. a Toyota, but it has nothing to do with the Lexus being technically superior.
- Mark